Don't know what oil was in it before we purchased, but was always dealer serviced. However, I more or less 'watched' as they did the work. If you can do it in 30 minutes you are truly a wizard. Takes that much time to remove the plastic covers and crap to access the rear of the engine and another 30min, or so, to replace them. Add in at least an hour of work at the highly inflated flat-rate and it all costs too much. If memory serves, the parts (five caps, o-rings & small bolts) were $322 and the rest was flat-rate labor and the misc crap one pays for at the dealer for service. In all, I was there for ~3 hour at $165/hr (think that is the dealer shop rate), so ~$495 plus parts is ~$900+. Been absolutely dry since for the last 45K miles & 5yr, so not a 'bad' investment, just one that I don't want to repeat if Mobil 1 was the cause. Guess I'll do the Mobil 1 in the CLK and find out. Now that I know the fix I'll purchase the parts and let my local guy do the work.