Originally Posted By: CATERHAM
Originally Posted By: vinu_neuro
Someone can ask BMW why they couldn't get it done with a 5W-40 or 0W-40 when Porsche and Ferrari do with their high-revving engines. But CATERHAM is wrong to go around telling people to use a viscosity that's substantially lower than what the engine manufacturer has specified. In addition to temperature, speed, load, composite surface roughness matter as well. You don't have all of that, BMW does. As oil is an engine component, everything else is designed with it as the specification. Believe it or not, there's more to tribology than viscosity index = good and zddp = good. Stick to what they recommend, certainly don't go two grades lower.
The more I've read on this subject, the more I've come to respect engine manufacturerss oil specifications for their respective engines. They do screw up now and again, but they're still the ones that produced the engine and have far more data and expertise than anyone here. It's fine being conservative, eg. shortening oci's if you're not comfortable, but I certainly wouldn't be aggressive in going below what they call for in important areas like viscosity.
And I think you should stick to what the manufacturer recommends.
My recommendations are always qualified and directed to those who first raise the question and are interested in thinking for themselves and less in dogma.
The oil grade one uses ultimately is not important, it's operational viscosity that matters. It is much more precise and safer to heed the test oil pressure spec's of a manufacturer in conjunction with oil temp's in determining how much of a viscosity safety margin one has for the way they operate their car. Not everyone is into that but some are.
Even with just an oil temp' gauge certain very safe assumptions can still be made. In the case of the M3 one is, if you're never seeing oil temp's above 212F then yes a premium 0W/5W-40 will more than meet the engines viscosity requirements.
And as far as being two grades lighter than spec', remember the FF is 3 grades lighter.
Doh, all those equations solved in models in papers published by OEM engine and lubricant manufacturers.got pwned by BITOG's CATERHAM. They forgot the oil pressure gauge!