Choosing the Right Oil Viscosity

Ever wonder if your choosing the right oil viscosity? Well the first step in choosing the right oil viscosity for your application is knowing and understanding what the Viscosity of oil really is. Viscosity measures the resistance to flow. Higher viscosity grades have more resistance to flow than lower viscosity grades. Oil gets thinner as it gets hotter. To determine the correct viscosity for an application you need to know the operating temperature of the oil in that application. Engines that run high operating oil temperatures require higher viscosity oil. Engines that run low oil temps require lower viscosity oil.

Years ago, it was common practice when choosing the right oil viscosity to use lighter “Winter” grade oils during the colder winter months, and then switch to a heavier oil for Summer driving. Accordingly, the number before the “W” reflects the “Winter” cold start flow of the oil. A lower number before the “W” indicates better cold start protection (lower viscosity base oil). The number after the “W” indicates the flow rate at 212°F (high temperature viscosity). Since most race cars don’t run in sub-zero conditions and oil temperatures vary above and below 212°F, the “Operating” viscosity of the oil is what is important – not the SAE grade.

For example, an NHRA Pro Stock engine, a NASCAR Sprint Cup engine and a World of Outlaws 410 Sprint engine have very different operating oil temperatures – 100°F, 220°F and 300°F. As a result, all three engines run very different viscosity oils – SAE 0W-5, SAE 5W-20 and SAE 15W-50.

As you can see, the operating temperature of the oil plays a major role in the selection of the proper viscosity oil. Using too high of a viscosity oil can result in excessive oil temperature and increased drag. Using too low of a viscosity oil can lead to excessive metal to metal contact of moving parts. When oil is of the correct viscosity, friction and wear are reduced.

It is important to keep clearances in mind. Looser clearances in the engine and oil pump require higher viscosity oil to maintain oil pressure. Tighter clearances allow for the safe use of lower viscosity oil for better cooling and improved horsepower. For more detailed explanations of how oil temperature and bearing clearances effect oil selection, please visit

The chart below provides a guide to choosing the right oil viscosity based on horsepower and oil temps.

For a application guide click here

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