Great site with lots of good info. Just purchased a new 03 Tahoe (yes an 03). Just dumped the factory fill 5W30 at 831 miles. Would have left it in longer but the truck had a Feb 03 build date and I was a little worried about condensation. Replaced the factory fill with Mobil Drive Clean 10W30. Two reasons. I live in mostly warm North Carolina and this is what the dealership puts in for oil changes. Their GM oil is Mobil 10W30 and it will probably go back there for regular changes. Would like to go for a heavier oil as temps rarely if ever drop below 20F. Only problem is the owners manual recommends 5W30 and says nothing above 10W30 and nothing without the starburst symbol or you void the warranty. I'm guessing a 5W40 or 15W40 would be the next progressively "heavier" weight multigrades. Correct???... I'm guessing I'll have to use 10W30 until the warranty runs out. Any ideas?