Question: what is the difference between 5/40 and 10w/40 oil?
Answer: The 5W40 is thinner than the 10W40 at lower temperatures. But when the temperature is higher, both motor oils similarly perform their intended functions (using the same viscosity). ... However, the main difference between these two engine types is that the 5W40 can work better than 10W40 in very cold places.