Can I Mix Synthetic and Conventional Oil?

The simple answer: Yes. There is no danger mixing synthetic and conventional motor oil; however, conventional oil will detract from the superior performance of synthetic oil and reduce its benefits.

The detailed answer: With synthetic lubricants continuing to grow in popularity, the question of whether synthetics and conventional oils can be safely mixed often arises.

I’m guilty of having mixed different types of motor oil with reckless abandon back in the day. A dash of synthetic blend to top-off my early ’90s Buick Century one month and maybe a shot of cheap conventional oil the next.

Though I’m sure performance suffered some, my engine never blew up as a result. So, yes, you can safely mix synthetic and conventional oil. In fact, synthetic-blend motor oil is simply conventional and synthetic oil already mixed for you. But, barring an emergency, it’s not a great idea.

Why it’s not a great idea to mix synthetic and conventional oil

All motor oil, whether synthetic or conventional, is a combination of base oils and additives.

Synthetic base oils are manufactured using a process that removes the impurities inherent to conventional base oils. Synthetics typically contain higher-quality additives, too.

So, in the end, conventional and synthetic oils are comprised of the same, compatible components – it’s just that the components in synthetics are much higher quality.

While you can safely mix synthetic and conventional oils, you’re doing nothing more than diluting the performance of the synthetic oil.

Find out here how to switch from conventional oil to synthetic.

You may not have a choice to mix the two in some cases.

If you’re traveling, for example, and suddenly find your engine low on oil, cheap conventional oil may be the only option at the gas station alongside the highway.

While it’s safe to top-off an engine that normally uses AMSOIL synthetic motor oil with conventional oil, we recommend changing the oil at your earliest convenience. In fact, AMSOIL does not support extended drain intervals where oils have been mixed.

When you’re using the best oil available, why add anything to it?

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Originally Published: April 13, 2015


    1. Hi Paula,

      It depends. What does the manufacturer recommend regarding maintenance and the type of lubricant to use?


    1. Hi Joseph,

      By “SxS,” I assume you mean side-by-side, or UTV. If so, yes, you can use synthetic oil. Check out our Product Guide for the correct AMSOIL synthetic lubricants for your machine.



    1. Hi Ronald,

      Yes. Make sure to use the correct viscosity, however. And make sure the oil is recommended for the performance specs listed in your owner’s manual.



  1. I was using a synthetic blend but switched to a full synthetic oil in my 2008 Honda Odyssey that has 150K miles. Since I’ve switched I have to closely watch my oil level as I usually have to add a half quart every 2 weeks. I don’t see any oil leaks on my engine or driveway.
    Why did this happen after switching to a full synthetic?

  2. I switched to synthetic in my 2015 Silverado 3500 dually. Been pulling a 44′ 5th wheel raptor with regular oil with no problem. After the switch at 37000 miles I started using oil. 1 qt after about 4000 miles. I’m a little disappointed

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