Not to belabor the thin vs. thick perpetual argument but...
First, as I've said many times along with many others here, there is an optimum weight for any given engine and conditions. Thinner isn't always better, nor is thinner.
To point one, I don't have to guess, I've measured the differences. I've seen cases where simply going from a 10W-40 to a 20W-50 would raise oil temps by as much as 20 degrees. But don't believe me, use the search feature on this very site. I think you'll find more people reporting lower oil temps with thinner oils than the other way around, everything else being roughly equal.
To the second point, I can only base this "startup rattle with thicker oils" observation on 2 engines in particular. A Toyota 3TC, and a Nissan VG30E. Both of these engines are solid lifter designs so the "sticky/worn lifter problem" doesn't wash. In both cases using thicker oils, especially in colder months caused noticeably more startup noise for a longer period of time. There was also a noticeable difference between the time full oil pressure was achieved, with the time advantage going to the lighter oils. It doesn't matter how much the oil "clings to the parts" if a load moves the oil away before adequate oil flow can replace it. Thinner oils flow better when cold, it's part of their definition.
The last point I think I should make, before we all go on believing what we did before is to point out the UOA's turned in by some of the new 0W-20 and 5W-20 oils. UOA's don't lie about engine wear and many if not most of the UOA's with the new 20 weight oils are excellent. We can all go on thinking they are too thin until pigs fly in small circles but if the UOA's show almost no wear, I'm going to say the oil is doing it's job...
[ November 25, 2003, 09:43 PM: Message edited by: jsharp ]
