I started using Mobil 1 In my 240 with 163,000 miles,it has 200,000 now with no loss of oil. I use it in two 940t~s 50,000 miles and 150,000 miles, 94 f150 with 83,000 miles none have oil leaks. Some times it will leak around seals that are considerably worn. Mobil 1 is real slippery more so than most petroleum oils. If there is any reason for a seal to leak like slightly higher crankcase pressures or gaskets that are on the verge of leaking, Mobil 1 will find it. On engines like this I always used conventional oil. You could most likley go back to the same oil used prior to Mobil 1 but it takes a while for some to stop leaking. Some do not stop leaking after clearing a path to less resistance. Synthetic oils also clean sludge build up around gaskets and seals that would in some cases help prevent oil seepage. This is evident after a few oil changes you start to notice the oil comes out much cleaner and the inside of valve covers looks cleaner. All disagreements welcome. ledfoot