With all the talk on thick vs thin (i.e. 20 wt or lighter oil) I've always thought there must be a definitive scientific answer. It's universally accepted that 90% of engine wear occurs at start-up when all oil (except qualifying oil) is too thick with the kinematic vis in the 100's or even higher when very cold. Many claim the ideal vis coincides with a 30 wt oil at normal operating temp. So that would translate to 9.3 cSt to 12.5 cSt. Widman goes along with this, suggesting wear increases in both directions outside of this range. With synthetic oil, many say it's no problem with oil temp's up to 300F and you don't need a to go to a heavier grade of oil. (GM 4418M). That translates into a viscosity under 4 cSt. Joe Gibbs Racing concurs with that, saying a vis as low as 5 cSt or even 4 cSt will not result in excessive wear and that of course is at full power. I think the answer is it depends. That yes, with the best syn oils and their advanced ad' pac's you can go as low as 4 cSt in engines with tight clearances and no particular lubrication issues. But the main issue remains having the lowest possible vis at start-up.