I took my 740 Turbo(210K) to the shop, not running smoothly. The mechanic showed me where the oil was leaking from the rear seal on the Mits Turbo and that it would have to be replace. He is checking on rebuilt prices. We got into a discussion on the turbo and he said to change the oil every 2k miles and that the kind of oil did not matter, I currently use Mobil 1. I thought Mobile 1 would give 5 to 10 k between oil changes. Have I been mislead by someone? What experiences have the members had? Ed