I have a 1995 Mitsubishi Montero with 150,000 miles on it. It is a 3.5 and has been running Castrol Syntec 5W-30 during the winter and 10W-30 during the summer. I have been reading and hearing that Castrol Syntec is not as good as Mobil 1. Also when I remove the cap were you add oil inside and around the cap there is some brown gunk, is that bad? It is in the shop getting some things done on the cooling system now. PS: How many miles do these cars usually last with that oil and the car has overheated 2-3 times when something was wrong with the cooling system.