Originally Posted By: cyclehead
I thought Bob recommended running the absolute thinnest grade oil you can find, that will provide 10psi per 1000RPM at full operating temps. He repeats over and over that 5W and 10W are all way too thick at balmy cold-start temps. Much less 0 degrees C.
You might be reading too much into it. Ideally, an lubricant would flow exactly the same at -40 as it would at operating temperatures. We all know, however, that isn't going to happen.
The point is that the thinnest 0w-20 is thicker even at 75 F than is a 20w-50 at operating temperatures. That's the point. The point is, in my opinion, designed to correct misconceptions like 0w30 being too thin where 10w30 is acceptable, and other things like that.
Ideally, an oil should be as thin as possible for cold starts and short tripping. Realistically, though, as long as one chooses a suitable oil for the expected temperatures, one should be okay. Also, some engines do have issues, big and small, with thinner oils. I wouldn't try to run the average BMW, Audi, or Benz on a high speed autobahn run with a 0w-20. It's probably too thin, and doesn't meet most specifications. Also, my old Audi 200 Turbo burned thinner oils, and by thinner, I mean anything in a 30 grade.
The issue is (and I'm guilty myself) not to obsess about temperature differences. Don't use 20w-50 when it's -40, but if it falls a hair below the 10w30 cutoff point on the oil chart, it's not the end of the world. A 0w30 or 5w30 will provide better cold start protection at all temperatures. In the grand scheme of things, what's it going to mean for the longevity of your engine? In non-extreme temperatures, probably not a lot.
In general, as CATERHAM has pointed out many times, to really know if you can successfully go to a thinner grade than recommended, you need to know your oil pressure and oil temperature.
