; ;I am often asked what the difference is between 5w-40 and 15w-40 Diesel oils? ; ; AMSOIL 5W-40 and 15W-40 diesel oils are multi-grade oils, meaning they provide good performance in high and low temperatures. AMSOIL 5W-40 and 15W-40 both behave like SAE 40 oils in high temperatures; however, the 5W-40 is thinner than the 15W-40 at cold temperatures.You can also expect ;a 1.6% increase in ;fuel economy with AMSOIL ;5W-40 over conventional 15W-40 Diesel Oils