Stupid oil question

[rpaco]

I'm about to do an oil change on my Xantia 1.9 TDSX and my brain has come to a halt over a very simple issue.

Mulitgrade oil gets thicker when its hot right? and thin for easy starting when its cold? Right?

So why does it say to warm the engine up before draining the oil

Or did I get that all wrong about multigrade oil?

[AndersDK]

Or did I get that all wrong about multigrade oil?

Not entirely - just reverse your theory and you are back on track

[rpaco]

So what is the point of multigrade oil then if it behaves like a single grade oil?

[jeremy]

It doesn't behave like a single grade oil. You want an oil that is thick enough to provide good lubrication when hot - as this is the temeprature of the engine most of the time. If you use a single grade - say 40 grade which is a common multigrade hot viscosity it will do a good job when the engine's hot - but when its cold it will be like grease! - and won't. You'd like something nice and thin - say 10 or 15 when your engine's cold - but that's too thin for hot running. Combine the 2 and it works rather nicely.

[AndersDK]

It does not. Any fluid gets thinner when hot - until it evapourates (the "thinnest state"). Opposite - any fluid gets thicker when cold - until its solid material (at 0degK).

Multigrade means its got a wider temperature operating range - where its not too thin or too thick to be used as an engine lubricant within the engine operating temp range.

If it was thin like water with a hot working engine - then the oil film would be too thin to protect - and engine friction too high in turn.
Opposite : if it is too thick when cold, the oil pump will give up the ghost and hardly make the oil flow at all. Again the engine would suffer from too high friction.
This is exactly the function of a single grade oil. Like a 30W oil for lawn mowers only used during summer season.

To keep the oil approximately at same useful thickness within a certain temp range - its made mutigrade. Like a 10w-40 specified for Citroen engines in temperate climates (like UK & DK).

At times you want a thicker oil. That could be in hot climates where you never get temps below 5-10degC. Then you'd use a 15W-40 or 20W-50, even a 25W-60.
This is also the solution if you have a worn engine with too high tolerances (i.e. too much space to be filled out with an oil film). Then you would try a thicker grade oil. In most cases it works miracles to a noisy tired engine.

Opposite : in cold climates with many real cold starts in deep frost, you want a thinner oil to be sure the oil will stlil flow easy at the starting temp. Like a 10W-30 or 5W-30, even a 0W-20.

[rpaco]

OK thanks I've found the answer elsewhere.

I've now found several technical explanations of multigrade oil on the inet.

But as to why drain it when hot, that came from Shell and seems logical:

In fact its to ensure that as much of the crap as possible is floating/in suspension and gets drained out with the oil. A matter of movement/agitation rather than temperature. While if its cold although the multigrade oil should still be thin, some of the crap may stay put on the bottom of the sump or other catchment places.

So thanks everyone

[Peter.N.]

The first 'multigrade' oil I remember was made by BP and called 'visco static' meaning it didn't get thinner as it got hotter, rubbish of course, but it didn't thin out as quickly with a rise in temperature as the 'ordinary' oil of the time.

[madmanbob]

OK. lets think of a 10/40 oil. At an engines normal operating temperature it would be the same thickness as a 40 grade, but of you reduce a 40 grade to 0C then it would be very thick and not circulate very well.
If you engineer the oil to be the same as a 10 grade when cold, it would be thinner than a 40 grade, circulate well and allow for better cranking speed on start up. As it warms a 10 grade would be getting much thinner. A multigrade that acts as a 10 grade when cold will thin but no where as much as a straight 10 grade. A 40 grade at 90C will still be thinner than a 10 grade at 0C but will still have the properties to maintain a constant oil film.
The ideal oil would be one that is the same thickness at 0c as it is at 100C, but it would then be a multigrade and I could not put numbers to that one. It could be as much as -30/90. So you see that the bigger difference in teh numbers the more constant the viscosity over differing temperatures.
That is one reason why high quality synthetics have wider ranges. I use a 5/50 in my motorbikes, it is of course, fully synthetic. For a semi synthetic I would use a 10/40 and a mineral would probably be a 15/40.
If you look at mobil oil they have the most advanced with a 0/60, but it is very expensive.

[JohnCKL]

Found this article. Its for aviation but does it apply to car engines?

Do straight-weight oils provide better engine protection?

http://www.eaa49.av.org/techart/str_oil.htm

[madmanbob]

Have not read the link, but no aviation is very different, I am an aircraft engineer by trade.
Most aircraft piston engines are either Lycoming or Continental, both are aircooled, both are a 1930s design and aircraft engines tend to have one start and then fly for hours at a constant throttle opening. If you think about it an engine in a cabin cruiser also has a very different working life than that of a car and would also be happy with a simple oil.

[Peter.N.]

Nevertheless it is a very interesting article John, apparantly showing that multigrade oil has its limitations, very 'back to basics'. My personal view and experienceof oil is that change intervals are more important than the 'quality' of the oil. I always use the cheapest oil I can find that has the required specification and rarely pay more than £6.00 per 5 litres, but I do change it every 5k miles, the last XM td I sold had covered 292k and a previous CX 266k. I have heard reports that many of the modern diesels are not lasting nearly as well and I think that could well be down to the long service intervals, good for fleet managers who dispose of them at less than 100k but not so good for the second owner.

[madmanbob]

I agree with what you say in principle but, and there is always a but, The quality of the oil makes the difference in the change interval.
For example; my first CX a 1981 CXD needed a 3000 mile oil change. The best diesel oil available then was API spec CC. The last CX I owned was a 1989 DTR Turbo2, same basic engine with a turbo and intercooler fitted. The manual said that the change interval was now 6000 miles, with API spec CD.
The reason? Because the addetive package was much better in a CD than in a CC and the main reason for changing a mineral oil is that the addetives break down. That is why people like Wynns and STP used to sell little bottles of oil addetive.
Modern oils are now upto API spec CF4, I believe, so the addetive packages will last much longer. A fully synthetic oil needs hardly any addetives so will last much longer, and I am happy to leave it in my HDI for 12000 miles. If I were to use a cheap oil, I would also change it every 3000 miles, but with fully synthetic at less than 25 euro per 5 litres here, it is not worth using anything else. There are other reasons for using a fully synthetic, but I'll not go into them here.

[stevieb]

My old 405 TD always liked Mobil 1 0/40.

Made cold starts a hell of a lot easier on the old girl compared to something like Castrol GTD 10/40.

Nothing as technical as what's already been said, but it was enough for me. And Mobil 1 being fully synthetic I knew it was going to last the full duration of the service intervals.

[Mandrake]

At times you want a thicker oil. That could be in hot climates where you never get temps below 5-10degC. Then you'd use a 15W-40 or 20W-50, even a 25W-60.

Hmm,

I've just noticed that I've put 20W-50 in my petrol Xantia at the last oil change, (Castrol GTX 20W-50) and I wonder if I've made a mistake ?

Mind you here in the northern most parts of New Zealand where I live we probably have a somewhat hotter climate than the UK. In summer (just passing now) we have had days exceeding 40 degrees, and in the middle of winter we usually get down to about 10 degrees, and on very rare occasions maybe 6 degrees now and then, but never below 0. Typical temperatures in autum and spring are around 20-30 degrees.

According to the owners manual 20W-50 is suitable for -10 to +50c while 10W-40 is suitable for -20 to +40c.

Have I made a mistake in my choice of oil or am I within the working parameters of 20W-50 ? Would 10W-40 have also been ok here or would exceeding 40 degrees in the peak of summer be outside the operating range for 10W-40 ?

Regards,
Simon

[AndersDK]

At times you want a thicker oil. That could be in hot climates where you never get temps below 5-10degC. Then you'd use a 15W-40 or 20W-50, even a 25W-60.

Hmm,

I've just noticed that I've put 20W-50 in my petrol Xantia at the last oil change, (Castrol GTX 20W-50) and I wonder if I've made a mistake ?

Mind you here in the northern most parts of New Zealand where I live we probably have a somewhat hotter climate than the UK. In summer (just passing now) we have had days exceeding 40 degrees, and in the middle of winter we usually get down to about 10 degrees, and on very rare occasions maybe 6 degrees now and then, but never below 0. Typical temperatures in autum and spring are around 20-30 degrees.

According to the owners manual 20W-50 is suitable for -10 to +50c while 10W-40 is suitable for -20 to +40c.

Have I made a mistake in my choice of oil or am I within the working parameters of 20W-50 ? Would 10W-40 have also been ok here or would exceeding 40 degrees in the peak of summer be outside the operating range for 10W-40 ?

Regards,
Simon

A mistake ?
I used to run on 20w-50 - on all my cars. Because when I purchase my cars they are generally (very) cheap for a good reason : a worn engine.
1.st job with a "new" car is always an oil + filter change. And my bet it would be like black ink when it runs out down there. For some reason many people here dont bother on oil changes. The use the red oil pressure warning lamp as : "need to add a bit of oil mate".

Imagine what happens to a worn noisy rattling engine - when it finally gets a thick and clean oil film to fill out those clearances.
* Silence * - and smooth running again.

Well Simon - I'd say using a 20w-50 on your (probably not so worn) Xantia engine in your climate - would pretty much equal me using a recommanded 10w-40 in a good engine in my climate.
Remember : over your ways with hotter climate - the "thicker" 20w-50 would be "thinner" anyway.
And I can NOT imagine you would have any cold start issues because of a thicker oil.