I am in agreement with the theory that someone is putting the
oil there..wife, neighbor..someone is messing with your mind. We did a similiar prank on a friend...he was bragging about fuel mileage...we added a little bit of gas to his tank each week...then we started siphoning some off for awhile...told him he was losing his mind and need to take math 101 again because the numbers just didnt match up.
He never did catch on.