What is 15W40 oil used for?

15W40 engine oil is most commonly used in vehicles during warmer months when the outside temperature is much higher. The number '15' indicates that this oil is thicker than alternatives such as 0W20; this in turn means it can last longer before it starts to thin.

What is the difference between 5W40 and 15W40 oil?

15W40 has a much higher viscosity grade than 5W40, therefore it's more suitable when driving in warm temperatures. On the other hand, 5W40 is the preferred option when the outside temperature is much colder due to it being thinner in viscosity.