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Really Quick Oil Question (hopefully)

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Hi all,


Done quite a bit of search here and elsewhere online but can't quite find a simple answer to this specific question and now very confused!


Am I right in thinking 0W/40 and 5W/40 will have same operating temperature properties but that the 5W will be more viscous at lower temps (and hence not flow so quickly or lubricate as well when the engine is cold).


If this is the case, why would you ever want to use the 5W instead of the 0W? That is, if the 0W lubricates better when cold, (but the same when hot) what is the upside to the 5W bit? I see lots of people are switching from 0W to 5W and not sure why...


I feel I really must be missing something!


Kudos for non brain hurting responses and apologies for anyone who is sick to death of oil convos.. They seem to generate quite a lot of controversy from what I've read!


Whilst we're at it..... what should I put in my 2002 C4S with 70K on the clock... I'm in the UK - going into winter! My local indy recommended 5W/30 but LN site says NO WAY...


Totally lost - help a newb!




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The real difference between the two oils is that the "base" oil is either 0wt or 5wt. VI improvers are then added to bring it up to the higher weight, i.e. 40. The "problem" is under heat and load the VI improvers like to shear which then reduces the viscosity of the oil, it breaks down. So.....the wider the viscosity range, the faster the oil will break down in service....relatively speaking.

The lower the viscosity the easier the oil flows... The higher the viscosity the higher the film strength, load capacity.

So......change it often. Lower viscosity when it's cold out, higher when it's hot and you're running hard. IMO

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The key here is "balance", namely which oil gives the best balance of cold starting, shear protection under heat, and show the ability to "stay in grade" (read still be the advertised oil grade after four or five thousand hard miles)


Both a 0W and 5W oil are low enough in viscosity to handle all but the most brutal sub zero F cold starts; in reality, the two oils are only slightly different in viscosity at low temps.  But as noted above, the 0W oil makes more extensive use of VI additives, so it tends to shear down faster (falling out of grade).  One area where most 0W and 5W oils differ is in film strength; 5W tends to be much better in that regard.


All things considered, I would look for a 5W-40 oil with ACEA A3, B3, B4 rating's with the highest ZDDP level you can find.  If you lived in a more moderate climate, I would even consider a 10W-40 that met the other criteria as well, as 10W oils tend to be even better than their 5W counterparts.

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Domiac, thanks! That's a very interesting read indeed. Judging hushed tones and sideways glances when oil starts getting mentioned.. I'm going to tread carefully! lol..


Two things strike me:


1. Sounds like we should all have oil pre-warmers which we turn on 30 mins before going anywhere ... Especially if it is actually the case that 90% of engine damage is done at start up! I know I'd use one - well, I would if it was remote operated over an IP connection from my iphone :-)


2. This suggests (I think.. and I am prone to getting things wrong) we should be adjusting our oil-type based on oil pressure readings at operating temperatures and certain rev ranges throughout our car's life. Eg. If our oil is up to temperature then at 3000 (or whatever) revs the pressure should be 3.5 bar (or whatever) if it is not, we should adjust our oil as the engine has changed with age.


Does anyone actually do this?


Even after reading this lengthy site.. I have to say, provided one is going to change the oil regularly it seems to make sense (to me at least) to use only 0W-XX oils and then whatever XX is required to get the oil viscosity right at operating temperature. I still can't really see why you wouldn't want the oil to be as thin as possible (and still actually too thick, in fact) at startup. (given the afore mentioned 90% thing).. Think I'll be sticking 0W-40 Mobile 1 back in mine.....imagine that.


Anyway, not trying to cause controversy at all, just quite a fascinating, complex and clearly contraversial topic!


Think I'll just go drive it! :-)

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