Greetings: In researching the issue of what oil (synth versus dino) and what weight (5w-30 versus 10w-30) to use I have decided to run Castrol Syntec but am debating what weight. I have a 2004 Chevy Avalanche with a 5.3L V8. I see that the 10w-30 has better High temp/High shear numbers than the 5w-30 (3.1 versus 2.9), however both are in the SAE range of min 2.9 to max 3.5 (ACEA A1, A5, ranges as well). My question is does a HT/HS number of 2.9, do an adequate job of protecting the bearings, etc. or would it be worthwhile to go to a weight with a higher number? Both Syntec weights pass the GM 6094M (light truck) and GM 4718m (Corevette) standards. Also, it is not particularly cold where I live (generally ambient temp in winter does not get much lower than freezing) but the manual states a preference for 5w-30. I elected to go with Syntec as the numbers look good on paper and with the UOA's I have seen. And for what I do, wear protection seems more important than temperature tolerance, hence Mobil1 made a very close second. Yes, AMSOIL is a great product, yes it has great numbers, yes it has a cool name, but the product with the great numbers is not API certified and the potential warranty issues rule it out for me. Thanx in adavance. Bob W.