My 96 Seville has the 4.6 Cadillac Northstar and I've run it on mostly M1 10W-30 and either PureONE, Bosch, or AC-Delco filters. It's at 96,xxx miles now. I probably only put 8-10k miles/year on it these days. When I didn't use M1, I used Pennzoil or Valvoline dino 10W-30's. Well, I've come to...