Bought a 2004 Mazda truck, 2.3L 4 cylinder DOHC Duratec motor. It has 50K miles on it. Previous and only owner took it to Iffy Lube every 5-7K miles and used 5w-20 Pennzoil YB. Inside of the valve cover is SPOTLESS. No varnish, no sludge, just shiny aluminum. Truck runs flawlessly Made a believer out of me. Pennzoil 5w-20 is good stuff!