What's the consensus on engine oil treatments such as: Slick 50? Dura Lube? STP Oil Treatment? Gunk Motor Flush? Gunk Oil Treatment? Marvel Mystery Oil? CD-2 Oil Detergent? CD-2 Oil Treatment? etc, etc? My stock 2L 16V has over 75,000 miles and I want to keep the car for a long time. BTW, small oil leak.