I have always ran synthetic in mine too. Mostly Mobil 1.
+1 on RP being expensive. Only place I can get it is Napa, and they want like $8/quart. I can get 6-packs of Mobil 1 quarts at Costco for under $30, and I can get 5 quart jugs of it at WalMart for closer to $20.
I know WalMart has a nasty habit of encouraging producers to rebrand a cheaper product as their own to sell there, but near as I can tell, the WalMart Mobil 1 seems to be the same stuff they sell everywhere else. I may hate the place, but it's hard to argue with the price.