To hawkster: the reason standard automotive oil is bad for quads has nothing to do with rpm's, there's cars that rev just as high as any quad out there. The difference is that most quads and motorcycles have a wet clutch that shares the same oil as the engine. Most modern automotive oils have moly and other friction modifiers which fill in all the little pits in the clutch plates which give it friction. After running auto oil for a while the clutch will slip like a greased pig.
Sidewayz: I started running Rotella 15w40 in my quads and motorcycle with no ill effects, no change in power or the way they ran at all. It's about 10 bucks for a gallon of it, lasts way longer than garbage moto oil like valvoline and yamalube, and contains no friction modifiers. People have been running this in motorcycles for a long time, and just recently the JASO MA rating started showing up on the oil jugs, the rating required my most motorcycle manufacturers. I noticed the same thing you are, a bunch of the cheaper oils i've tried were extremely thin and broken down after a few months, my dual sport gets exceptionally hot at times because of idling in traffic with no air flow, and within 400 miles the oil wasn't even dirty but was scary thin, almost like gasoline, so I had to change it. Valvoline was one of the worst, and I have a suspicion that Yamalube and Suzuki's oil is coming from the same plant because it was equally garbage, just more expensive. Silkolene has been the best "budget" oil i've used, but it's increased to nearly $7 a quart here so it's still a pretty premium price for some friggin oil.
I'm not meaning to start a fight because thats usually what happens with oil questions, but that's my 2 cents and take it for what it's worth. I just got the shits of spending 7 bucks a quart for half decent moto oil. Rotella Triple-T for the win!