The 5 and 30 refer to the relative viscosity of the oil when it is cold (5W) and when it is hot (30W). A 5W-30 oil behaves like a 5W thin viscosity oil when the engine is cold, and it behaves like a 30W thicker viscosity oil when the engine is hot. A very thin viscosity is undesirable in a hot engine. In very high temperature situations, an even higher viscosity range, up to 50W, might be called for. Always go by the engine manufacturers recommendations.
Chat with our AI personalities