Simple Guide, How to add Additive to Gearbox C4 II, B7, 2013, Citroen

[frer8833]

Guide on how to top up with gearbox oil additive.

This very simple DIY method may be applicable to similar models.

Overall effect from using oil additive in both engine and gearbox: smoother acceleration with more power and car sounds more lubricated.

I added 6% of an oil additive called Mecarun P18 to the gearbox (an EGS6 but it works for manual too) because it is a car with old, but lifetime, gearbox oil.

Mecarun states that 6% is for harder driving conditions so I emptied a second bottle into the engine oil reservoir. In older cars 6% is better because they accumulate stress and wear from real world driving, the effect is also more noticeable for the driver in terms of sound, engine smoothness, fuel economy.

Oil additives like Mecarun P18 and Mannol can reduce friction over 20%. Even adding just 1% ester based friction modifier to 4L of 5w20 engine oil reduced the friction by 12.45% and improved fuel economy by 2.33%¹. Mecarun P18 may give 5-10% better fuel economy according to a reseller². In comparison using 5w20 engine oil instead of 5w30 reduces friction 1-2% but with risk of insufficient lubrication.

There are similar additives like Mannol 9829, 9929, 9903, and 9091 that form a resilient oil film on metal parts but that is different than Mecarun P18 which creates a sub-micron thick, super slippery, and permanent metal coating which is much better. 6% Mecarun P18 + 3% Mannol additive may be even better as it can improve the old gearbox oil (but try not to overfill the gearbox, max is 2.2-2.3L).

How I poured additive into gearbox (pics):

First I detached the lower end of the air intake pipe to make more room for my hand. Then I inserted my hand, palm up, into the area where the gearbox oil refill lid is located and turned my palm down, pulled off the lid and turned palm up again with lid between my fingers and pulled my hand out. A plastic bag covered my arm against scratches with holes for my fingers. The pump nozzle was shortened to improve the flow of the thick fluid.

The gearbox oil specification is PSA Peugeot/Citroën B71 2330. 2.1L +/- 0.1L refill after draining (semiauto/manual). B71 is C4 II.

BTW, on this C4 B7 model I prefer 205/60R16 tyres for better cushioning compared with OEM size 205/55R16.

¹"Experimental Research on the Impact of Lubricating Oils on Engine
Friction and Vehicle Fuel Economy" https://www.atlantis-press.com/article/25837920.pdf
²"This improvement in engine performance contributes in particular to a significant reduction in fuel consumption. For driving primarily in cities, the savings achieved are estimated at 5 to 10% depending on the vehicle." https://additifs-auto.com/hyper-lubrifi ... oteur.html

[akerolf]

Can you specify little further how to locate the fill plug for gearbox oil? My car is C4 2011 automat.

[frer8833]

Hi akerolf.

The filler plug is almost directly below the black battery tray. In the picture, you can follow the transparent PVC hose (next to the syringe with red additive fluid) back to the oil filler cap/lid. You’ll need a flashlight to see the lid clearly.

Without checking your VIN number you either have the semiauto version with small knob actuator handle in the center console (automated manual transmission) with gear shifting paddles behind steering wheel or the manual version with regular "big" gear shifter and no paddles.

If it is a C4 II (B7), then the gear oil spec is B71 2330, which is same for both semiauto and manual. There should still be 2.1 litres in the gearbox since car left factory because gearboxes have low temperature and don't consume oil unlike car engines. If the gearbox oil level is low because of leakage, you'd get a low gearbox oil error code in the OBD2 scanner. So I assume your car still has 2.1L in the gearbox.

There is no dipstick to check gearbox oil level. If you drain the gearbox you can see how much it is by collecting it in a measuring bottle or weigh it on a scale and pour back in an equal amount through the top refill (venting mount) hole. If you overfill a little bit, you may be able to use a syringe and hose to suck the surplus back out through the same top refill hole.

Do you have a problem with your gearbox, any symptoms? If not, I recommend simply topping up with 125ml of gearbox oil additive. This is more convenient than draining and refilling.

You can write a clear note in the vehicle’s service book or maintenance log stating the date, mileage, exact amount (125ml), and product name of gearbox oil additive. This helps prevent accidental overfilling by future owners or mechanics.

(BTW older cars are susceptible to rust, so when car is dry I inspect the vehicle from all angles, including underneath. On rusty areas I first spray brake cleaner, wipe clean with a cloth, and then apply 2–3 coats of 1K touch-up paint from spray can, using the car’s color code, and finish with 1-2 coats of 1K clear varnish from spray can. This saves the car from becoming destroyed by rust.)

[akerolf]

Hi frer8833,

Tanx for your replay!

[frer8833]

You're welcome, akerolf!

After adding the leftover fluid in the additive bottle, the engine now contains 9% Mecarun P18, up from 6%, which is a clear improvement in how the engine feels (more efficiency, more fuel saving). The gearbox still has its 6%.

Mecarun recommends 8% in both engine and gearbox, for most extreme driving. In tests it increased torque/hp, lowered drivetrain temperature and noise, resulted in metal to metal 85% friction reduction*.

Mecarun P18 contains antioxidants which can extend the life of the engine/gearbox oil if it is added at the oil change.

A good cheaper alternative is Mannol 9929, 10% in engine and 6% in gearbox (or extract/drain a little gearbox oil first to ensure not overfilling and add 10%).

My simple method avoids this complicated drain and refill disassembly shown in this YouTube video, which states 1.9L but should be at least 2L.



*"Épreuves d'endurance ou conditions extrêmes : jusqu'à 8% du volume d'huile", "Boîtes séquentielles à usage intensif : jusqu'à 8%", "Augmenter le dosage à 7-8% for rallye and tout-terrain for conditions extrêmes (poussière, variations de température)" and "Réduction de friction Jusqu'à 85% selon tests tribologiques spécifiques". Auto Detailing Reims,
https://web.archive.org/web/20250425120 ... itesses-1l

[frer8833]

Update: A mechanic drained the old gearbox oil and refilled it with fresh oil. The gearbox now feels smoother and quieter than it did with the old oil plus an additive.

Mileage was 270,000 km, so that “lifetime fluid” was probably a bit overdue

It can still feel a bit jerky sometimes when creeping in 1st gear, but that seems more related to clutch behaviour than the oil. It might be improved with a clutch bite point relearn/readaptation.

I haven’t tried adding an additive like Mecarun P18 to the new oil yet, but it might improve things even further.

If you change the gearbox oil every 5 years, you probably don’t need an additive. But if you only change it every 10 years, and especially if you do a lot of high speed motorway driving, frequent mountain driving, regular towing, long runs with heavy loads, or if the engine has been tuned for higher torque, an additive might be worth considering.

[frer8833]

To correct several critical misconceptions regarding the EGS6 (MCM/BVMP6) transmission, and to rectify some of my own earlier comments, here is the definitive breakdown of PSA product descriptions, fluid volumes, and additive chemistry for vehicles like the Citroën C4 B7 and Peugeot 3008:

First, it is a dangerous myth that a low mechanical gearbox oil level from a leak will trigger an OBD2 error code or dashboard symbol. The mechanical gearbox housing completely lacks an electronic level or pressure sensor; it can leak until bone-dry and self-destruct without warning. The warning symbols and error codes people experience related to "low fluid" actually stem exclusively from pressure loss in the completely separate electro-hydraulic actuator unit (which uses a distinct hydraulic fluid, such as Tutela CS Speed, not the gear oil).

Second, regarding oil capacity for the EGS6: The official factory specification dictates a refill volume of 2.1 liters (±0.1L) after a standard drain and refill, and 2.2 liters (±0.1L) following a complete overhaul. (Note: Newer ETG6 evolutions from 2013/2014 onward utilize a physically redesigned, smaller casing revised by PSA to a 1.7-liter capacity to optimize efficiency). When incorporating the Mannol 9903 additive, combine 2.0 liters of fresh gear oil with the 50ml additive. This total volume of 2.05 liters safely matches the standard maintenance target while remaining well below the maximum dry overhaul capacity.

Third, regarding Mannol 9929 (Ester Additive): Mannol’s official website states that 9929 is an "additive to engine oils and to oils for mechanical transmissions". While highly fluid and possessing a high viscosity index, its core chemical package is optimized for internal combustion environments, containing ZDDP and heavy detergents designed to prevent deposits of lacquers, resins, and sludge. At a 5% to 10% mixing ratio, adding these components to a dedicated PSA B71 2330 gear oil risks altering the precise friction coefficient required by the synchronizer rings. If the friction baseline shifts, it can delay synchronization windows, causing the automated TCU shifting logic to register sluggish engagement or actuator stress. Furthermore, these engine-specific detergents can actively compete with the transmission's native Extreme Pressure (EP) surface additives.

Fourth, regarding Mecarun P18: While Mecarun P18 is also marketed for universal applications ("for engines, manual and automatic gearboxes"), its chemistry differs fundamentally from Mannol 9929. P18 is not an engine oil concentrate with detergents; it is a highly refined hydrocarbon-based friction modifier engineered to form a micro-thin boundary tribofilm on metal surfaces under heat and pressure. It does not introduce engine-specific cleaning detergents or risk thickening the fluid layout. However, in a computer-controlled robotized transmission like the EGS6, applying a highly potent anti-friction surface treatment at a standard 4% ratio still introduces an operational variable. Because the TCU monitors synchronizer engagement down to milliseconds, making the synchronizer rings excessively slick can cause them to slide rather than grip properly, potentially triggering delayed synchronization codes.

Fifth, regarding Universal API GL-4/GL-5 LS (Limited Slip) Oils (e.g., Mannol Unigear 75W-80): Many universal gear oils list the PSA B71 2330 specification alongside "API GL-5 LS" on the bottle. While modern additive chemistry can technically balance GL-5 shear protection with synchromesh compatibility using inactivated sulfur, the "LS" (Limited Slip) designation introduces a critical mismatch for automated manual gearboxes. Limited Slip additives utilize potent friction modifiers specifically engineered to allow wet clutch packs inside a limited-slip differential to slide smoothly and prevent chatter. In an EGS6 gearbox, the synchronizer rings act as mechanical brakes that must instantly grip the gear cone to match shaft speeds within milliseconds. Introducing a fluid chemically modified to prevent friction surfaces from grabbing directly opposes the transmission's synchronization requirements. Any extended synchronization slip can trigger immediate shifting faults, shift logic delays, or actuator timeouts.

Sixth, the best approach for the EGS6 gearbox is to avoid universal multi-vehicle lubricants, engine-targeted packages, or combined API GL-4/GL-5 formulations. While a standard 2L refill of the older PSA B71 2330 specification works (such as TotalEnergies Traxium Gear 8 or Fanfaro Max 7), the ultimate setup for a high-mileage unit is 2L of a fluid meeting the newer, more advanced PSA B71 2315 specification, topped up with 50ml of transmission-specific Mannol 9903 Getriebeoel-Additive. This combination ensures exceptionally fast actuator response during freezing winter mornings thanks to the lower viscosity and superior base oil of the 2315 standard, while the Group V esters from the 9903 provide a permanent, high-strength boundary layer on the gear flanks when hot. Prime examples for the 2315 base are Shell Spirax S4 G 75W-80 or Ravenol PSA SAE 75W-80, which are pure API GL-4 oils; these should not be conflated with API GL-5 fluids carrying the same approval (such as the Liqui Moly variant), as pure GL-4 chemistry is critical to preserve proper synchro friction.

Finally, ignore GearboxList (gearboxlist.com); there is no internal fluid filter, and while the core hardware is incredibly robust and can exceed 300,000 km on factory fluid under gentle driving, regular maintenance is vital to counteract the architecture's native flaws. In real-world operation—particularly in platforms like the Citroën C4 B7 and Peugeot 3008—the EGS6/ETG6 TCU logic is notorious for becoming jerky, hesitant, and twitchy over time. Because the automated system relies on millisecond-accurate sensor feedback (selector position, shaft speed) and high-cost electromechanical actuators, any degradation in fluid friction parameters directly accelerates these drivability issues and increases component stress.

To maintain factory synchronization windows, ensure noticeably smoother gear shifting, and prevent premature wear and humming on the internal differential and output shaft bearings, a fluid exchange every 200,000 km is recommended. Note: This applies to the mechanical gear oil, not the separate Tutela CS Speed hydraulic fluid used in the external actuator reservoir.

Crucial Driver Operating Guidelines to Prevent Transmission Stress
Gear Selection Errors During Driving

1. Engaging Reverse (R) before the car has completely stopped
Problem: Forces the reverse gear components to mesh while rolling, which creates severe mechanical grinding and wear.
Solution: Always wait 1–2 seconds after the car is completely stationary before shifting to Reverse.

2. Engaging Forward (A/M) while the car is still rolling backward
Problem: Forces the clutch and internal gear trains to abruptly counteract the car's rearward kinetic movement, causing unnecessary stress.
Solution: Use the footbrake to stop all movement before changing travel directions.

3. Rocking the car back and forth when stuck in snow or mud
Problem: Rapidly throwing the shifter between forward and reverse causes severe clutch slipping and risks overheating the electronic clutch actuator.
Solution: Keep the car locked in either 1st or Reverse, apply gentle, steady throttle, and use external traction aids if the car remains immobilized.

4. Constantly downshifting manually with paddles at very high RPM
Problem: Forces the synchronizers to aggressively brake the input shaft speed to match high engine speeds, accelerating normal wear.
Solution: Let the TCU execute downshifts automatically during deceleration, or manually downshift only at lower, reasonable engine speeds.

Gas and Pedal Handling

5. Aggressive throttle inputs during an ongoing gear shift
Problem: Stomping on the gas pedal exactly when the robot is interrupting torque confuses the software and can lead to a jerky, poorly timed engagement.
Solution: Maintain steady, predictable pedal pressure and allow the robot to complete its automated shift cycle smoothly.

6. Flooring the gas pedal directly from a standstill
Problem: Forces the automated clutch to slip aggressively under heavy load to prevent the engine from stalling, causing rapid clutch wear.
Solution: Roll away smoothly for the first few meters to let the clutch fully engage before applying wide-open throttle.

7. Pumping the accelerator pedal during acceleration
Problem: Confuses the transmission control unit (TCU), causing it to hesitate, hunt, and cycle-shift between gears unnecessarily.
Solution: Hold a constant, linear foot position so the software can properly calculate, select, and lock the correct gear.

8. Pressing the brake pedal while simultaneously applying throttle
Problem: Confuses the transmission ECU, leading to conflicting commands where the system tries to engage the clutch while the brakes resist, overheating the clutch.
Solution: Drive strictly with one foot, ensuring completely separated brake and throttle inputs.

Cold Start Management

9. Starting the engine instantly without letting the hydraulic pump build pressure
Problem: Cranking the engine immediately after entering forces the hydraulic system to work before the system has reached its optimal operating pressure.
Solution: Unlock the car or open the driver’s door to trigger the automatic pump priming, turn on the ignition, and wait a few seconds for the buzzing sound to completely stop before cranking the engine.

10. Aggressive driving and high RPM before the engine and gearbox are warm
Problem: Heavy acceleration before the fluids reach operating temperature prevents optimal lubrication and increases wear on moving parts.
Solution: Drive gently, keeping engine speeds below 2500 RPM for the first few minutes of every journey.

Overloading and Towing

11. Towing a trailer heavier than the car’s maximum rated capacity
Problem: The extreme weight forces the automated clutch to slip excessively during take-offs and generates fatal heat buildup inside the gearbox clutch housing.
Solution: Strictly adhere to the manufacturer's towing limits or use a conventional manual vehicle for heavy hauling.

12. Driving with heavy loads up steep inclines in a gear that is too high
Problem: Refusing to downshift bogs down the engine and forces the clutch to endure maximum torque load under low RPM.
Solution: Manually drop a gear using the paddles to keep engine RPMs in an efficient range, ensuring proper power delivery.

Protecting the Drivetrain against Road Hazards

13. Speeding over speed bumps and deep potholes while applying throttle
Problem: When the front tires momentarily lose traction and suddenly regain grip upon landing, a severe torque shockwave is sent backward through the driveshafts into the differential.
Solution: Complete all braking well before the hazard, and coast over bumps with your foot off the pedals so the drivetrain is torque-free during impact.

14. Accelerating hard on uneven surfaces (gravel, washboard roads, ice)
Problem: When the tires repeatedly lose and suddenly regain grip, it sends rapid torque-snaps directly into the differential and internal gear train.
Solution: Ease off the throttle and maintain a steady, low-load speed over loose or icy ground.

Parking and Diagnostics

15. Relying solely on the transmission to hold the vehicle on steep inclines
Problem: The EGS6 gearbox does not possess a mechanical parking pawl (P). If turned off in N, it rolls freely. If turned off in A/R, the car's mass pulls against the internal gear teeth and dual-mass flywheel.
Solution: Always apply the Electronic Parking Brake (EPB) firmly to absorb the vehicle's weight, and leave the gear selector in A (or R) as a secondary mechanical backup.

16. Ignoring transmission warning lights and continuing to drive
Problem: Disregarding a "Gearbox Fault" message can turn a minor, easily fixable electronic glitch or low fluid level into major mechanical damage.
Solution: Pull over safely, turn off the engine, and have the system diagnostic-scanned (preferably with a Diagbox/Lexia scanner) immediately when a fault light appears.

Tires and Differential Protection

17. Driving with mismatched tire dimensions on the driven axle
Problem: Using a wrong-sized tire alters wheel speed readings, which tricks the ABS/ESP sensors and forces sudden, jerky, or mistimed automatic shifts.
Solution: Only use identical tire sizes across the driven front axle.

18. Driving with severely uneven tire pressure on the front axle
Problem: Creates a constant rolling speed difference between the two wheels, forcing the internal differential gears to work continuously even when driving perfectly straight, creating heat.
Solution: Check and equalize front tire pressures regularly.

City Driving and Traffic Jams

19. Creeping in heavy traffic
Problem: Continuous creeping causes the EGS6 gearbox (Citroën C4 B7 2013 / Peugeot 3008 2012) to slip the clutch under 6–7 km/h in 1st gear and 13–15 km/h in 2nd gear. This creates a dangerous overlap where clutch slip and engine lugging occur at the same time, generating extreme heat and destructive engine vibrations.
Solution: Switch to Manual mode (M) and lock the gearbox in 1st gear, or use the downshift paddle while in Automatic (A) mode to force a downshift to 1st gear. This eliminates clutch slip and protects against engine lugging. Allow a gap to form ahead, then accelerate past 10 km/h to lock the clutch completely.

Advanced Driving Techniques

20. Holding the car stationary on an incline using only the accelerator pedal
Problem: Forcing the engine to hold the vehicle's weight on a hill without pressing the brake pedal causes the electronic clutch to constantly slip at high friction, causing extreme heat and burning the clutch lining within minutes.
Solution: Use the vehicle's built-in Hill Assist feature, or apply the handbrake until you are ready to accelerate away cleanly.

(Rustprevention: Older cars are highly susceptible to rust, so when the car is dry it should be inspected from all angles, including underneath. To protect the vehicle from rust damage, first scrape away loose rust, apply a rust converter to the remaining pores, and apply a base layer of 2K epoxy primer directly after the rust converter to create a superior, impenetrable barrier compared to standard 1K bases. Next, apply the 1K touch-up paint matching the car’s color code in multiple layers without sanding between them, as sanding the wet or fresh basecoat will ruin the metallic pigments. Once the color coat is dry, seal the repair by applying 2–3 wet, cross-coated layers of 2K clear varnish via spray or brush, allowing only a few minutes of flash-off time between coats without any sanding so the droplets can fuse into a solid, glossy plastic shield with no microscopic gaps; this completely blocks new moisture and road salt from penetrating from the outside. Only after the final 2K clear coat has fully cured for several days may you use an ultra-fine P2000–P3000 wet sandpaper to level out any dust or orange peel before polishing it to a mirror finish, while an elastic underbody coating should be used underneath the chassis instead of 2K clear varnish.)