I have a E430 2000 with 50k miles. My problem, I think is I noticed a knock when starting up in the morning, only and goes away with in a couple minutes. I was told it could br the Air Injector Pump. I use Mobil 1 0-40w full sythetic. Is this serious or common with this type of engine. I baby this engine and never drive it hard. Also how often do others change there oil.