What is the difference between putting in 15w40 and 20w50 oil?

From the viewpoint of behavior when cold, oils with a lower SAE before the W will behave better at low temperatures. In other words, a lubricant with SAE 15W-40 will behave better when cold than a lubricant with SAE 20W-50. There is also a difference in the behavior when hot. Grades with a higher SAE after the W tend to generate thicker films of lubricant and to protect better since viscosity is necessary to protect the mechanical parts that are in contact. However, excessive viscosity at high temperatures could cause more internal friction and poorer engine performance. Our recommendation is to use the lubricant recommended by the manufacturer. You can find this in your vehicle manual or on our search engine.