Using 5W40 oil instead of 15W40 depends on the specific requirements of your vehicle. The numbers in the oil classification refer to its viscosity, which indicates how thick or thin the oil is at different temperatures.
In general, the "W" stands for winter, and the first number represents the oil's viscosity at low temperatures, while the second number represents its viscosity at high temperatures.
A 5W40 oil has a lower viscosity at low temperatures compared to a 15W40 oil, which means it flows more easily in cold weather. It also maintains a stable viscosity at higher temperatures.
If your vehicle's manufacturer recommends 15W40 oil, it is likely because the engine is designed to work optimally with that specific viscosity. Changing to a different viscosity, such as 5W40, may alter the lubrication properties and potentially affect the engine's performance or warranty.
It's always best to follow the manufacturer's recommendations for the oil viscosity specified in your vehicle's owner's manual. If you have any concerns or questions, it's advisable to consult with a certified mechanic or contact the vehicle manufacturer for guidance specific to your vehicle model.