i forgot where i saw this, but NYC did a test where they put brand new engines in around 300 cabs and drove them for 100,000 miles or so. each motor was given a specific oil and a specific milage to go. ex synthetic, regular oil at intervals of 3k 5k 10k and 15k. they then removed the engines at 100k miles and tore them apart to see what the difference was and guess what. there was no definitave proof that 1 brand was better or that any oil change interval was better. just go with whatever the dealer say. the only problem i see with that is that oil filters dont last as long.

dont quote me on that, but its something along those lines. but i still change my oil every 3 months :P