I dont think 508 is as available as you think. Chances are slim to none you will find 508 anywhere except the dealer or possibly Napa. Check for yourself. Back to my studying now. Rocket science doesn't teach itself.
Originally Posted by MCompact
It's not rocket science- use what the manufacturer recommends, period. Odds are the oil with the correct approval(s) is available at AA, AZ, and WM. If the shop doesn't know what the correct spec is, find one that does. Simple.