Can you mix two different motor oil viscosity types to make your own custom viscosity? Is it OK to mix two motor oils of the same viscosity type? What about mixing different brands of engine oil or mixing conventional and synthetic oil? Can you mix motor oils for gasoline and diesel engines? Let’s talk about everything related to mixing motor oil.

First off, a reader asks: I’m looking for an engine oil for my drag race car engine from your Dominator series. But I would like something between the 10W-30 and 15W-50 products that are offered. Can I mix the Dominator 10W-30 and 15W-50 to make (roughly) a 12W-40? Does this work?

Answer: Mixing two different motor oil weights to make your own custom viscosity does not work quite as simply as it sounds. There a couple of unknowns and even some potential complications that have to be considered.

As the additive formula used to construct each viscosity (10W-30 and 15W-50 in this case) can utilize different chemistry, there simply is no way to project how well the mixture will actually work and whether the longevity of the oil mix will be affected.

While the viscosity of the mixture may result in an engine oil somewhere in the SAE 40 range, where the “W” (winter viscosity) will end up is anyone’s guess. Again, different chemistry can be used to “build up” the viscosity of an engine oil, so mixing two from different viscosity ranges is not likely going to deliver the result that you desire.

AMSOIL Dominator Synthetic Racing OilAnother uncertainty is the shear stability of a mixture of two different weights. As the chemistries of the two oils can vary, there is no way to predict how well the blend will stand up to shearing forces and extreme heat. While it may not be a disaster, the safer choice would be using one type of motor oil that was formulated to perform at its best.

Our recommendation would be to use either Dominator 10W-30 or 15W-50 on their own. If you are insistent on SAE 40 oil, consider AMSOIL Synthetic Premium Protection 10W-40 Motor Oil.

If you are in a pinch, you could certainly top-up your engine oil level with an oil from a different viscosity range. It may not be the end of the world, but we are not endorsing the practice of brewing your own custom motor oil formula for complete oil change intervals and especially not for racing purposes.

Can You Mix Motor Oils Of The Same or Similar Viscosity?

What about mixing a 5W-30 and a 10W-30 in your car’s engine? This is better than mixing two completely different grades as both are an SAE 30 weight. But as the winter weights are different, it may not be a major problem (but it also may not be optimal). Adding a 10W-30 to a 5W-30 should be fine for a top-up, but is not ideal for a complete service interval.

What About Mixing Different Brands Of Motor Oil?

For example, assume that you are using Quaker State 5W-30 in your engine and for whatever reason, you need to add oil during a long trip and only Valvoline 5W-30 is available. This is perfectly fine in a situation where you need to top-up your oil level and only a different brand is available. Again, as these oils may have a slightly different chemical make-up, it’s best not to mix two brands for an entire oil change interval.

Can You Mix Synthetic and Conventional Oil?

There may some conventional oil left in your engine when you switch over to synthetic oil and this is no problem. The mix of synthetic oil and conventional oil will not create any sort of chemical discord in your engine.

But when it comes to mixing in larger amounts, the same caveats involved in mixing brands have to be applied here as well. There can be different additive chemistry at play, so adding conventional motor oil to synthetic oil (and vice versa) is fine for a top-up. As AMSOIL’s literature suggests for their synthetic oils, adding regular oil to their long-life synthetics will shorten their lifespan.

Can You Mix Diesel Engine Oil And Gasoline Engine Oil?

While gasoline and heavy-duty diesel engine oils perform similar duties, the operating environment of diesel engines and gasoline engines is quite different. Most diesel engine oils meet the industry specifications for gasoline engines, but most gasoline engine oils are NOT suitable for diesel engines. They simply are not equipped to deal with the soot and high pressures of heavy-duty diesel engines.

The exception is some heavy-duty motor oils that are designed for both diesel and gasoline engine service and some European formulas tailored for gas or diesel cars.

So you can top-up a gasoline-powered car or truck with a diesel engine oil, provided that it meets the proper industry specifications. It is definitely not a good idea to top-up a heavy duty diesel engine with a motor oil designed only for passenger car/gasoline engine service.

Conclusion

The prevailing theme here is that you can mix many all types of motor oil in a pinch, but it should not be done as a general practice. Motor oil makers work very hard to create the right balance of chemical additives, so toying with this balance is generally not going to be of any benefit unless you are in dire need of a top-up.