I just want to add that most of the problems I'm seeing with oil here in TX are sludged-up engines. Good synthetic is a lot more resistant to sludge than any dino every could be. Changing oil more often will help on most engines, but engines like the VW/Audi 1.8T and other over-stressed European car engines will sludge up with dino oil unless you change it weekly...
Somewhere I remember reading about Pennsylvannia oils......Pennsoil and Quacker State, etc having a wax material base that caused buildup problem with the higher temps here in the warmer states. I was a certified master mechanic in an earlier life, did street rods and a lot of drag racing....everything was a heavy oil in those days (Valvoline, Marvel Mystery Oil, etc). Hot Rods never made it to 100K miles for other reasons, anyway. Have noted the change to very light oils and synthetics. Think the CNC tooling, machining, and controls are much superior, so the engines are much better fitted, and the engines last much longer.