Which oil?

[XantiaMan]

Which means that the cold bit only is relevant when starting and the hot bit is going to be the most important.

I may be wrong but my research suggests that practically any engine oil gives adequate protection when hot. I would have thought, considering most engine wear occurs at cold startup, that the opposite would be true?

Admittedly I have more reading to do but it's an insteresting subject!

Not so, if the 2nd part of the oil rating is too thin, the oil film will not be enough to protect the engine under extreme operating temperatures. This is why many sports cars run a thicker oil, BMW M stuff use a 10w 60!

[MikeT]

I may be wrong but my research suggests that practically any engine oil gives adequate protection when hot. I would have thought, considering most engine wear occurs at cold startup, that the opposite would be true?

Not so, if the 2nd part of the oil rating is too thin, the oil film will not be enough to protect the engine under extreme operating temperatures. This is why many sports cars run a thicker oil, BMW M stuff use a 10w 60!

Agreed on your theory, though I'm not likely to see racing temperatures - well, maybe occasionally but it would be very short-lived! 8)

[Bren]

Go to Morrisons and buy Carlube 15/40 Diesel oil - it costs £7.99. I think it is rated at SH(suitable for turbo's) which exceeds citroen specifications. It is a mineral oil and I have used it for as long as I have had my xantia with no ill effects. I also use it for my mother-in-laws' V6 Granada - diesel oil has more detergents to combat the sulphur that finds its way into the oil on a diesel engine. I do a change every 4000 miles. It is better to use a cheaper oil and change it at shorter intervals.

A good oil filter is a must - I used to use purflux but got fed up with paying rip - off main dealer prices. Mann or Hengst (from euro car parts) are just as good.

As an aside, many years ago I had a Vauxhall Senator( when petrol was much cheaper). I switched from 15/40 mineral oil to a 10/40 semi - synthetic. The engine was much quieter when cold, but most of the oil was going out of the exhaust! I was constantly topping it up - much too thin for that engine.

[steelcityuk]

I generally use the Citroen recommended stuff from GSF. For the sake of a couple of pounds I won't scrimp on oil.

Personally I believe that all these incredably long service intervals will mean alot of newer cars will have knackered engines not long after the warranty has run out, I don't understand how the same engine used by different manufacturers can have vastly different service intervals - could it be that one of them is hoping to sell more cars because the servicing cost are lower?

I change mine (well the XMs) twice a year, before Winter and after Winter. I also put high power magnets on the filter too.

Steve.

[coopertron]

I just bought some Total Havoline 3 15/40W Diesel Oil, 5L from the Morrissons Petrol Station down the road, for £8.99, which seemed cheap enough to me.

Hope this helps someone...

[XantiaMan]

I just bought some Total Havoline 3 15/40W Diesel Oil, 5L from the Morrissons Petrol Station down the road, for £8.99, which seemed cheap enough to me.

Hope this helps someone...

Asda sell the same oil for slightly less, its a good price for a branded oil. My personal fav is Shell Helix 10w 40 Diesel, but thats about £13-14 quid.

[MikeT]

diesel oil has more detergents to combat the sulphur

Here's a thought. Does SVO produce sulphur?

That aside, I'm hoping to find and try 10W/30 if it's not expensive. Tesco are always selling 15W/40 for diesel and 10w/40 petrol both for £8, I have the former in the engine now and it hasn't lost any in 5000 miles.

[XantiaMan]

I'm hoping to find and try 10W/30 if it's not expensive.

Please dont use 10w 30! Its far too thin for an XUD, and will run past the piston rings when hot! Ok it might not, but its an old engine design that doesnt run uber close tolerances and will make no use of the thinner viscosity, it will be more likely to leak and be noiser, without any benefit to engine protection or mpg improvements. 10w 40 is fine for a car thats done over 100k, anything else is over kill.

[steelcityuk]

It's interesting to note that the diagnostics expert in Car Mechanics (Dean Birch?) quite often recommends replacing the oil and filter with the exactly specified one before even trying to fix running problems.

I would think that too thin an oil in the XUD could leak past the turbo seals and slowly coat your intercooler with oil which will lead to a drop off in performance not to mention increased emissions.

The oil bible is a good site - http://www.carbibles.com/engineoil_bible.html

The recommended Total brand from GSF should only be about £10/£12.

Steve.

[myglaren]

That link is very interesting steelcityuk.
A lot of it is pretty common knowledge (hopefully anyway) but the bits that aren't could save a lot of hassle and a fair amount of cash too.

[MikeT]

Please dont use 10w 30! Its far too thin for an XUD, and will run past the piston rings when hot! Ok it might not..

True, it might not. 8) That's what I'm hoping for but I appreciate the warning and will keep a watchful eye out for evidence.

I would think that too thin an oil in the XUD could leak

I agree but what is "too thin"? If a 10W/40 has the same viscosity 10W/30 in normal driving it stands to reason that it cannot be too thin.

The oil bible is a good site - http://www.carbibles.com/engineoil_bible.html

A bit long winded and interesting to note that page is supplied with a plea for funds to buy him a bike - if he's so smart he can earn it IMO!

The first issue I have with this article is he claims that it is safe to mix different oils when the general consensus seems to be not to do that as each oil is made with different additives and mixing them can have undesirable effects.

Secondly, he appears to be writing from a viewpoint of 1980's built cars - I don't know when my diesel engine was designed/built?

Thirdly, he states that diesel oil is a very effective flushing oil for petrol cars and advises a 15W/40 which is too heavy for my 1993 petrol car.

Eventually I find a part that I can agree with - he recommends using the thinnest oil possible. 8)

(As an aside, it's interesting to note he states that US mandate says a catalytic converter should last 150,000 miles) Something else I can agree with 8)

[steelcityuk]

How can 10w/40 and 10w/30 be the same? Surely if it was this simple then they would be labelled the same? Also if it was so straightforward why do car manufacturers specify an exact oil for their engines?

What is too thin? I'd say something that wasn't recommended by the manufacturers. If too much gets past the seals in the turbo the engine can start to run on it until it goes pop. Highly unlikely but due to oil getting past turbo seals/bearings (due to overheating) it has killed many a Laguna DCi. Also as mentioned if you start to get a nice coating of dirty oil in your induction system it will reduce efficiency and could lead to a MOT emissions failure.

As for the engine (XUD) it was used in 405s which date back to the 80s.

At the end of the day it's your car.

Steve.

[MikeT]

How can 10w/40 and 10w/30 be the same?

If I may, let me turn this around and ask you how you think they are different and more importantly, why? Bear in mind I didn't say they were the same viscosity (otherwise they'd have the same number ratings), I said under normal (i.e. public road) driving conditions in the UK.

Surely if it was this simple then they would be labelled the same?

Agreed. So what's the difference?

Also if it was so straightforward why do car manufacturers specify an exact oil for their engines?

I can't answer that but I can tell you that the oils we use today are far superior than the oils specified at the time of manufacture. Secondly, it is not unknown for manufacturers to change their recommendations over time. Thirdly, I believe that one manufacturer do not (dare not?) actually specifiy/recommend an oil by it's viscosity because of the difference in performance of oils according to application.

What is too thin? I'd say something that wasn't recommended by the manufacturers.

Do you now approve of my choice? My Citroen handbook recommends a variety of oil viscosities and the thinnest one is 5W/30.

At the end of the day it's your car.

...if I want to destroy it, you mean? But that's why I'm posting here - to be aware of that possibility, and some good arguments have come out of it. But so far, I haven't read any justification not to use an oil rated at 5W/30 (or lower for synthetics) other than out-of-spec seals/gaskets etc and, of course, cost of oil. Additionally, I'm hoping (though probably 130,000 miles late) to reduce engine wear considerably and improve efficiency, power and consumption.

[XantiaMan]

My Citroen handbook recommends a variety of oil viscosities and the thinnest one is 5W/30.

Yes, but for what ambient temperature? This is the key factor and the reason why oil is rated at 2 set temperatures, one from cold (the W part) and the second part when hot.

A 10w 30 will technically be the same viscosity at its cold rating as a 10w 40, but at its 'hot' tested temperature, say its 120C, the 10w 40 will be thicker than the 10w 30.

New cars use thinner oils because the tolerances are tighter. Closer ring gaps to reduce blow by, which reduces emissions, and the thinner oil also improves mpg. The oil galleries and pump are designed to push this thinner oil through the engine.

So you want to put a 0w 30 oil into an engine originally designed for a 15w 40 (at a typical ambient temp range of 10C to 25C). The oil pressure will be reduced, which means less oil available where its needed from a cold start. The engine will be louder because the oil will not cushion the wider gaps and tolerances of the older engine.

When hot, the oil film will be thinner which will increase the risk of the bearings becoming hotter, they will wear quicker as a result. As the oil gets hotter, oil pressure will drop even further as the oil pump with its big galleries designed for a thicker oil struggles to pump this water like substance around and up the engine.

I am explaining worse case scenario here. The more miles the engine has done, the worse it gets, as the oil pump will have worn, the running clearance of the bearings is larger and the ring gaps have increase because of bore wear.

Oil is a personal choice, but the gains you make from dropping the oil viscosity from that recommend does not give enough advantages to make it worth while, especially in what is effectively an old, well used diesel engine never used on the race track. I dont know why you are so concerned about reducing engine wear on an engine that is well run in and will see another 100k plus no doubt if regular oil changes are performed. If you want to increase performance look at other ways because oil will not give you enough to notice!

The subject of oils is extremely complicated, so keep it simple and dont deviate massively from the original recommended oil.

[steelcityuk]

As I said your car your choice, whether it blows up or not I guess you'll find out.

Play the word games if you will but why go on a forum asking questions then get spikey because you don't like the answer?

Seems to me you taken offence at what was just logical thinking.

I will keep my opinion/advice to myself in future.

Steve.