Originally Posted By: supton
It's been raised before, and it's a long standing question. IMO, unless actual engine changes are found(look up the crank, rod and main bearing p/n's, then looking up the crank and rod p/n's, and seeing if that the same parts are being used across this time span), odds are very good that the OEM did some testing, and found "acceptable" wear rates going to a lighter oil. What that means to us is... ?
My opinion is, it doesn't matter. 0W20 might wear the engine faster, but it'll likely make 300k without issue. Just look at all the engines making 200k+ without issues, and all the sub-200k in the junkyards for reasons other than worn bearings. 5W30 might cost a pinch in mpg (may be impossible to actually measure) but it will hardly damage the engine--and you would have to run it past 300k to find out if it was lower wearing. Which many don't.
I used to have two fears about running other than specified 0W20: VVT, and timing chain wear. From what I can tell, chain guides will wear regardless of oil type; and badly designed chains will also wear out, regardless of oil type. But "good" chains won't. Meaning, it's a design problem, not lubrication. It will go the miles, or it won't, but it won't be a fault of the oil. Secondly, the VVT has to work at "cold" engine temperatures, down to -40F / -40C; and in order to do that, it must work with... very thick oils.
If I lived down south, with high ambient temps, I'd be quite tempted to run xW30, especially if the prior years indicated it. I would have no qualms though about running xW20, especially if I got it at a good price. In the end, it probably doesn't matter.
Your comments on chain wear is a little off. Chain and guide wear is accelerated with oil abuse. I use 0-20 and both engines are very clean. and there is no sign of chain-guide problems. Deposits will restrict oil flow in the area of the guides and cause premature failure. 20-30 probably makes no difference, but oil abuse will. In my opinion, using a quality synthetic oil just makes since.