ford told me every 5000 miles, but mine was getting dirty at around 4000, so i changed it. i have been told to stay with the manufactures recommendations on oil, they have engineered the motors for the specific weight. i run 20/50 in my 69 cobra with the 454 ford engine, but if i used the 20/50 in my 08 that it wouldn't oil right, to heavy of oil,