Engine Oil - 2.7 - what do you use
- Thread starter AntB
- Start date
Wow - what a great thread this has turned into... Great info and experience.
So - if I read this correct - I think the general is that a 5w40 all be it a great oil is potentially the wrong grade for this engine..
I still have 10 ltrs of Mobil Motorsport 15w50 oil, from what I read - this will actually be quite a good oil for my 2.7 8v 944 then??
- not sure that they make Mobil Motorsport anymore tho??
Cheers A
So - if I read this correct - I think the general is that a 5w40 all be it a great oil is potentially the wrong grade for this engine..
I still have 10 ltrs of Mobil Motorsport 15w50 oil, from what I read - this will actually be quite a good oil for my 2.7 8v 944 then??
- not sure that they make Mobil Motorsport anymore tho??
Cheers A
Neil Haughey
New member
Ant I switched to Mobil 1 15w-50 a few years back in my S2 when I was living in Weymouth about 4 or 5 years back. I found that sitting in traffic in the summer I was loosing about a pint every 1K miles and would often see low oil pressure. Note that back then we had some hot summers, probably the hottest driving conditions you will ever see in the UK as we had many weeks in the 30's. I switched to the 15w-50 and oil consumption dropped to maybe a pint every 10K miles. On a race track the car will still loose roughly a litre an hour but never actually see any smoke. Engine has 174K miles on it still on original cams. I have been a fan of the thicker oils for some time now and will be moving to maybe 10w-50 from the original 5w-40 in my 968.
sawood12
New member
This will happen on every engine due to gravity surely. Also the oil will drain out of the crank bearings into the sump. That is the whole point of using an Ester oil (I personally wouldn't use anything else) - these are polarised and so cling onto the surfaces providing an element of protection. I chose my oil grade to maximise cold start protection - don't forget all your engine wear happens during the first 15mins or so of every journey you make because of the oil draining into the sump when the engine is off.
The problem with thick oils is that the oil actually provides less protection. If you ever drag your finger along the bottom of a bowl of porridge (once you've eaten it of course) you will leave a dry line. That is what is happening with an oil that is too thick. If you use a thinner oil the oil (or porridge in this analogy) is dragged back behind your finger and leaves the surface wet - hence better protection. So as long as you have a good flow of oil being provided to the moving surfaces then you are maximising your protection. Also you are not getting as good a flow of oil if it is thicker - you will get less flow of a thicker oil at 5 bar pressure than a thinner oil (back to the sucking milkshake through a straw analogy).
Clearly an oil does need a certain level of thickness or we'd all be filling our sumps with 3 in 1. The reason I think oil has to have a level of thickness is to resist against the oil from being squeezed out from between the two moving surfaces. Again the developments in modern fully synthetic oils massively improves this characteristic in thinner grades of oils. So you are trying to balance out wanting a nice thin oil to improve oil circulation around the engine and maximise wear protection, and having an oil with some level of thickness to actually allow the oil to do what it is supposed to do.
I'm not sure what grade you need to get to before it is too thick. In reality there is probably little real difference between a 5w or a 10w. I don't even know if these numbers are linear i.e. is a 5w half as viscous as a 10w??
In terms of fully synthetic oils - I see no good reason NOT to use a fully synthetic in any car (other than proper classics). There is not one benefit that a semi synth (which are just a marketting con as they contain such a minute % of synthetic content) or mineral oil has over a modern fully synth oil. Modern fully synth oils withstand temperatures better (important in turbocharged cars), are more resistant to breakdown due to contamination of water, fuel and products of combustion, are more stable (i.e. maintain their properties longer - oils become acidic over time which eats away at your engines internals, fully synths get acidic much more slowly). The reason why modern cars can go 20k miles between services is due to fully synth oils.
Scott I dont think its right to talk about thick and thin oils when you bring multi grade into the mix as they change characteristics with temperature. You could have two oils with different viscosity when cold but the same when hot for example. Like you say with oil sticking to surfaces theres more to oil than thickness. Afaik synthetic oils also have low surface tension compared to semi/mineral oils, this makes them more prone to leak.
germanmetal
New member
A couple of things to remember,
1/Most of our cars have huge mileages,not just for a sports car but also any car(so must be doing something right!!)!I was once told it really matters not what oil you use but that it is changed regularly(obviously not varying wildly).Be it 15w-40/20w-50 or even 5w-40 change the oil regularly!
2/In the 80's oil technology was not as superior as is today,to get a decent multigrade now,synthetic base stocks are used to counter the 'sheering' that oils are subjected too.This allows a large 'gap' if you like between the 1st and 2nd figure.This was not really possible with earlier oil so was a kind of make do attitude.
Personally i use a very good 5w-40 fully synthetic,but that is because it is free to me.If i was buying; good 10w/15w-40/50 changed regularly would be my choice.
Incidently i use no oil with 166k!
1/Most of our cars have huge mileages,not just for a sports car but also any car(so must be doing something right!!)!I was once told it really matters not what oil you use but that it is changed regularly(obviously not varying wildly).Be it 15w-40/20w-50 or even 5w-40 change the oil regularly!
2/In the 80's oil technology was not as superior as is today,to get a decent multigrade now,synthetic base stocks are used to counter the 'sheering' that oils are subjected too.This allows a large 'gap' if you like between the 1st and 2nd figure.This was not really possible with earlier oil so was a kind of make do attitude.
Personally i use a very good 5w-40 fully synthetic,but that is because it is free to me.If i was buying; good 10w/15w-40/50 changed regularly would be my choice.
Incidently i use no oil with 166k!
Copperman05
New member
I agree with scott on this, I'm changing to fully synthetic oil after running on semi 10W-40 for the last year. I remember when the last time I changed to synthetic on my motorcycle I actually felt an improvement in engine performance and never went back. It seems that oil is one of those subjects where many people seem to have an opinion that is very loosely related to fact. Most people who seem to know about oil all seem to agree that synthetic oil is the superior option. Synthetic oil has much greater tolerances than that of mineral oil, it can withstand much higher heating, and most critically it can withstand much higher compression than mineral oil, i.e. when it gets squeezed by tiny tolerances between metal and metal synthetic oil is still working protecting the metal surfaces where mineral oil would not.
I really can't see why I would use mineral or semi oil anymore for anything other than price. I personally haven't seen any increase in oil consumption with full synth oil and I wonder why increase consumption is immediately considered a bad thing? (apart from cost) Could it not be because fully synth is perhaps getting in between those small tolerances, lubricating where standard oil isn't and then getting burned off during combustion? And if that is so does it mean it's a bad thing or a good thing? My point is, increased consumption perhaps in itself shouldn't be looked at as a negative but that it could be indicative of an actual benefit. It would be interesting to hear the scientific viewpoint on this....
Edd
Yes its late....I work nights!
I really can't see why I would use mineral or semi oil anymore for anything other than price. I personally haven't seen any increase in oil consumption with full synth oil and I wonder why increase consumption is immediately considered a bad thing? (apart from cost) Could it not be because fully synth is perhaps getting in between those small tolerances, lubricating where standard oil isn't and then getting burned off during combustion? And if that is so does it mean it's a bad thing or a good thing? My point is, increased consumption perhaps in itself shouldn't be looked at as a negative but that it could be indicative of an actual benefit. It would be interesting to hear the scientific viewpoint on this....
Edd
Yes its late....I work nights!
Copperman05
New member
On what basis do you make that assumption?
Edd
Copperman05
New member
That synthetic oils are unsuitable.
sawood12
New member
I am mainly talking about the cold viscosity aspect as that is what most of this discussion seems to have focussed on - i.e. some people are speculating that a 5w is too thin. That may well be the case, but I would certainly say that moving to ever thicker oils under some misguided idea that you are somehow getting better protection is a dangerous game to get into. When the oil is upto working temp and viscosity a 5w40 is identical in is protection capability as a 10w40 15w40 etc.
I'm pretty sure you can get fully synths in pretty much any multi grade combo you need can't you? You can get 0W through to 20W and 30 through to 60 and any combination of the two sets. You can also get fully synthetic single grade oils for race purposes and other specialist purposes.
I am not aware that fully synths are more prone to leak. Isn't a leak more likely to be due to a 20yr old seal long past it's expiry date? After I replaced a few leaky seals in and around my engine i'm completely leak free even with fully synthetic oil.
At the end of the day though how sensitive our our engines to oil grades within reason? On the basis of the number of people across all the forums and the huge variety of oils people choose to use, engine failures that are directly attributed to the use inappropriate oils are basically so rare they are practically unheard of. I cartainly havn't read of a single engine failure or a single engine requiring a rebuild due to the use of the wrong grade of oil in my 6yrs or so of 944 ownership and scanning across many forums covering most of the free and civilised world. I've heard some hearsay and pubtalk about it and anecdotes about someones friend of a friend of someones uncles cat, but no-one has actually posted to say that their engine has just expired because they used a 10w40 instead of a 15w50.
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