Yeah, I saw a test where someone ran M1 5w30 up to (I think) 18,000 miles before they decided to change it out. The oil anaysis said it was still okay for a little while longer. That's insane. That's the test that indicated the oil got 'better' with it's age...
Mine was the 4.3 V6 - 2000 Blazer. I change the oil every 5K. The motor started to really lack power on the highway when it got up there in age. At 90K miles, it was burning almost a quart a oil change. At 95K miles, I started it one cold morning and thought a rod was going to come out the pan - I've never heard a motor rap so loud and stay together!
The stuff I was reading about was all in the GenIII motors. Mostly high performance (6000+RPM) LS1 stuff. Everything I've read indicated that 5w30 was generally too thin for heavy duty or high performance use.
I really gotta dig up that info... I read it a while back...