5w40 Oil Viscosity is a thinner oil that is best for cold weather conditions. 15w40 Oil Viscosity is a thicker oil that is best for hot weather conditions. 5w40 Oil Viscosity is less viscous than 15w40 Oil Viscosity. 5w40 Oil Viscosity flows more easily at low temperatures. 15w40 Oil Viscosity flows more easily at high temperatures. 5w40 Oil Viscosity is ideal for vehicles that operate in cold climates, while 15w40 Oil Viscosity is ideal for vehicles that operate in hot climates.
There is no definitive answer to this question as the best oil viscosity for your vehicle will depend on a number of factors, including the climate you live in and the type of driving you do. However, in general, 5w40 oil is thinner than 15w40 oil, so it may be a better choice for vehicles that operate in warmer climates.