Question: what is the difference between 5w-40 and 15w-40 oil?
Answer: Viscosity is a fluid's resistance to flow. ... Both will have a viscosity of a 40 grade. That cannot be said for both oils during the dead of winter. The 5W-40 will have a lower viscosity when the engine is started than the 15W-40; consequently the 5W-40 will start easier and lubricate quicker in the cold than the 15W-40.