Can I Mix Synthetic and Conventional Oil?

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.

Let’s take a closer look.

Can I mix oil types?

Can I mix oil types?

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 Buick Century one month and a shot of cheap conventional oil the next.

While oil performance probably suffered, my engine always remained in one piece.

That’s because 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 to mix oil types.

Pouring AMSOIL XL Oil into engine.

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 how to switch from conventional oil to synthetic.

What if you have to mix oil types?

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?

AMSOIL Signature Series

Updated. Originally Published April 13, 2015.


  1. Years ago my Instructor in auto mechanic when asked about mixing brands of oil (was before synthetic oil came along) said to always wrap the oil container with a rag when adding to car then car will not know what kind of oil you are using and will not care.

  2. Ted Winter; I read your comment about using zinc mixed in your motor oil. Personally I buy Rotella, because it has many good characteristics and for what you’re doing I would recommend the Rotella. I use a quart in my SS Monte Carlo after about 50,000 miles. Also helps with deglazing cylinder walls, and is a fairly high amount of detergents.

  3. I have a 2005 4runner -since new. It has 150,000 miles and runs great.

    I tried synthetic oil in one oil change and got drips immediately, so I went back to conventional and change at 5000 miles.

    The drips stopped completely 10 months after going back to conventional oil.

  4. GM will tell you one quart in 4,000 miles is not excessive. I have 2 of the 6.3 trucks, both use about that much from new to now (about 150,000 miles). If you were changing oil by the “change oil” light or sooner and not checking the oil regularly with the dipstick, that loss would not cause the low oil level light to come on before a change. Many people when changing oils worry about added consumption and start checking the dipstick, revealing consumption that was always there.

  5. Went to have my oil changed in my Truck, (100,000 miles) wa told I needed high mileage oil. High mileage Oil?

    1. Hi Benjie,

      “High-mileage” oil is the fastest-growing type of oil today. It’s designed specifically for vehicles with (typically) 75,000 miles (120,000 km) or more. As engines age, seals can dry out and parts can wear if motorists haven’t be using a good synthetic oil and maintaining their vehicle. High-mileage oils contain extra additives designed to condition aging seals to help prevent leaks. Their viscosities also fall on the upper end of the spectrum to help seal the widening clearances between metal parts that are wearing over time.

      While you don’t have to use high-mileage oil, especially if you’ve been using a good synthetic oil and performing proper maintenance, there’s no harm in doing so. If it were me and I knew I’d been properly caring for my engine, I’d thank the technician but continue using the oil I’ve always used (AMSOIL, of course for me).



    2. Hi Mileage motor oil contains an additive for flat tappet camshafts. So if you are running that type of engine you should be running that type of oil. I collect old garden tractors. They are designed to run straight 30 weight oil with the old Kohler or Onan engines. I am using a break in oil that comes with that zinc additive in it. My 95 Jeep is designed for 10w40 with flat tappet cam. My snowblowers will probably get 5w30 hi mileage.

    1. Hi Frank,

      In general, it’s best to use an oil specifically formulated for the application in which you intend to use it. We can’t speak for another oil manufacturer, so contact the oil manufacturer and see what they recommend.



  6. Actually all conventional oil is going to synthetic blend, Since I’ve switched I have to closely watch my oil level as I usually have to add a half quart every 2 weeks.

  7. Actually all conventional oil is going to synthetic blend. Mixture of conventional and synthetic.

    1. Been driving for 50 years, and have always mixed or changed brands. Never had an engine go bad.

  8. I currently have an older skidoo mineral oil in my unit. (Skandic) It’s a pao based oil. It’s OEM for the year (2003) but of course is no longer made other than the small supply I have on hand. Is the amsoil severe gear (75w140) compatible with this BRP pao base oil or would I need to do a full oil swap to ensure no tarring etc.

    1. Hi Marcus,

      SEVERE GEAR is compatible with synthetic (PAO) and mineral oils; however, for best performance we recommend trying to remove as much of the old fluid as possible before switching to AMSOIL.



  9. It’s a great Oil I use ever, I’m using it more than 1 year, it really works great.

    Highly recommend for every vehicle owner.

  10. Wow lots of dumb questions I hope I don’t sound stupid but have posed this question to auto parts oil reps and they can’t give me strait answer. Ok there are about 5 syn oil base stock which one is best to worst and why and what brands types there are no labels on oil bottles.

  11. One reason could be that the synthetic “eats up” the gaskets and other components. Regular oil does not have that ability and actually the debri it produces helps the sealing of the engine from oil leaks. This is generally stated to be the case for older engines. Your car is pretty new and that should not be the issue; it is doubtful, but it may be also possible that your engine was not built specifically with synthetic in mind.

    1. Hi VK,

      I’m not sure why you think synthetics “eat up” gaskets and other components, but it’s not true. Synthetics pose no harm to gaskets, seals or other components. If it were true, would automakers increasingly recommend synthetics in their vehicles? Also, while engines are designed with a specific viscosity of oil in mind, they aren’t built to use a specific type of oil (e.g. conventional or synthetic).



    2. Synthetic oils most certainly do not “eat up” gaskets and other components, maybe try changing the oil once in a while, that seems to be the problem here.

    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.



  12. 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?

  13. 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

    1. That’s easy. The problem is you’re driving a Silverado. Are you sure it’s not just leaking out of the pan like every other GM HD?

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