If a manufacture recommends a certain weight of the oil for the engine, lets say 5W-40 and instead 5W-30 was used, would the gas mileage of the engine increase as the piston does not have to do as much force going up the walls and down?

Would the engine noise decrease?

I understand that the manufactures recommendation is based on how the engine was built and the gaps between the internals of the engine but what would happen?

1 Answer 1


According to this article, oil viscosity does effect fuel mileage.

This particular test was done using a diesel engine in a controlled environment by Chevron engineers.

They used 15W-30 and data collected against the use of 15W-40 showed a %0.8 percentage difference between the two with the 15W-30 winning out on the better mileage.

The delta was attributed to the work of the oil pump on thicker or thinner oils. The piston moving within the cylinders would, more than likely, be unaffected by a slightly thicker or thinner oil as the piston is not pumping the oil, it's merely using it as a lubricant. The friction co-efficient between the two is nominal.

It is possible that you may have a slight decrease in engine noise, especially with straight cut gears within the motor that tend to be louder. This would typically only apply to very high performance cars though.

  • This only makes sense, considering heavier oil will be harder to push around, thus taking more energy to get it where it needs to go. Sep 20, 2016 at 21:55
  • Just like a heart and blood thinners used to reduce it's work after a heart attack. Sep 20, 2016 at 21:57
  • So do people in races usually go for a low viscosity oil to gain that extra power or is it very minimal that they do not bother? also would the oil pressure in the engine decrease because of having a lower weight oil?
    – method
    Sep 20, 2016 at 22:04
  • 1
    Oil pumps are designed to maintain oil pressure despite the viscosity. Most engines have a spring loaded pin that will release excess pressure in the oil galley system. Yes, racers attempt to use the oil that will give them the best lubrication and and viscosity balance. Sep 20, 2016 at 22:20

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