GM says to use 5 or 10w30 in most of their car and trucks. Some Fords call for 5w20. If you use an oil not recommed for your perticuler engine it will void the warranty...why? Mobil 1 0w40 will protect as good or better in an LS1 V8 then 5w30 so why won't GM honnor the warranty with this oil in the engine? Fords owners book say any other oil then 5w20 will viod their warranty. If an oil will protect well why not keep the warranty in effect? GM, Ford, Dodge, and all the imports know that oil other the what is in the owners book does protect as good if not better in certain conditions.