Originally Posted By: dennishiip
OK then,but doesn't the 0W and 5W winter rating mean it is not thick? I live in California and drive down central valley in 110 degree heat and was thinking the 40W would help in those conditions. I also wonder abut oils shearing down to lower viscosity after some miles rendering them less effective.
No the 0W and 5W winter rating has absolutely nothing to do with how thick the oil is once up to operating temp's, it's the second number of an SAE multi-grade oil that determines that.
When a manufacturer spec's an oil grade there is no advantage of using anything heavier because they have already factored in everything that can happen to reduce the operational viscosity of the oil, including oil shear, fuel dilution and high oil temp's. For example Ford spec's 5w-20 for their 400+ HP Mustang GT and even for track use on the hottest days possible Ford specifically does not recommend using a heavier grade oil, doing so would actually be counter-productive.
BTW, regarding German cars, running 40 and 50 grade oils may be common practice in Europe for a number of reasons, but that doesn't mean it's automatically required and the 30 grade is spec'd for most makes such as Mercedes, Audi, VW and BMW. And regarding BMW, 0W-20 is even spec'd for their new cyl turbo, see below:
http://www.generaloils.net/2014-02-12_AIS-InfoFuchs_TITAN-GT1-EVO-0W-20_EN.PDF
Without trying to figure out how heavy an oil you need to use for yourself based on oil pressure and oil temp's, running the lightest oil grade recommended by the OEM will provide you with all the high temperature extreme use protection you'll need and them some.