The biggest reason to change your [normally aspirated] engine oil frequently is short trips. If you are filling your crankcase with lots of blow-by moisture, unburnt, and burnt gasses... the acid created can quickly use up the additives in the oil used to fight them.
7500 miles using any API-SL oil (dino or syn) should be OK if all your trips are above 10 miles, more during the winter.
[ February 02, 2003, 11:07 PM: Message edited by: S2000driver ]