eeeh. i am about to buy some engine oil for my car. i've always been using this generic brand, autolab. At the garage i work for, we always use autolab too, unless a customer wants castrol GTX which we charge more. Now, would switching to a more expensive oil make my engine run smoother and produce more power? If theres hardly any gain from switching from my trusty old cheapass autolab to high roller GTX or Pennzoil, then i guess sticking to any SAE approved oil is good enough.

It shouldn't make that much of a difference if any. On the backs of oil containers there is a letter rating. I think the current rating is SM. A year or three ago it was SL. The lower down the alphabet you go, the more obsolete the oil is. If you look at the back of the oil sold for cars 1988 and older it will say something like SH, not sure exactly. So the next generation of engine oil will be SN.

Without getting all technical, that is the general idea.