I've only been a mechanic for a few years now but I can tell you NEVER EVER use Penzoil. I can smell a penzoil engine- too much parafin in the **** and it causes a lot of grime and burt buildup on every damn part of the engine- a pain to work on.
Mobil and Royal Purple are great oils- no doubt about that. If you're like me though you'd rather not spend $25 on an oil change if you don't need to. Valvoline conventional is from all I've seen the best low priced oil out there. I'll sometimes ask a customer what oil they use if its a damn clean engine under the valve cover, and it is always Valvoline. And those cars also are coming in at around 150,000 miles for routine maintenace like timing chains and belts and such never having had an internal problem. In the winter, if you have a harsh one, go for the pricey stuff like Mobil One or Valvoline Synthetic, however if it stays above 50F stick with standard 10W-30 Valvoline. It just makes sense.