When is it OK to Mix Oil Grades?

There’s a lot of hearsay and mystery surrounding the fluids we put in our cars. It’s always “don’t mix this” or “don’t use that” or “that oil’s no good” etc etc. As a result, doing your oil changes and even something as simple as topping off your engine oil has become a confusing and stressful affair. 

It can be especially worrisome if it’s on your classic car, or any car with sentimental value. How can you know the difference between what will keep your engine protected and what could possibly do damage? Can you mix grades? What about synthetics vs non-synthetics, or even different brands? That’s what we’ll be talking about today, and hopefully put your mind at ease with some best practices. 

Mixing Grades of Oil

Credit: SIGAUS – www.sigaus.es

Say you’re in the middle of nowhere, on a road trip you’ve been looking forward to for months. Filling up at the gas station, you check your oil level (like a good little boy/girl), and you find that it’s LOW. What’s worse, you’re not even sure what grade and type of oil is in there in the first place. Your smartphone is no help since, after all, you’re in the middle of nowhere and don’t even have a single bar of service to look it up. 

What do you do now?

There’s a few guidelines, but most engine oils will be safe to use. Especially when only topping off, there’s no harm in using a different grade. Most cars on the road use between 0w-16 and 10w-40. Almost all Japanese cars will be 0w-20 or 5w-20, with American cars using a lot of 5w-20 and 5w-30. There are of course exceptions, but the general guideline is all we’re after right now.

Mixing grades really isn’t harmful, but straying too far outside of the grade your engine was designed for could become an issue if you do it too often. Too thick (higher numbers), and it may not flow well enough to properly lubricate. Too thin (lower numbers), and it may burn off. My advice here would be to stick with a middle of the road grade like 5w-20 if it’s available. 5w-30 will also work just fine. 

Head into that gas station, grab some oil and top off your engine.

Thanks for reading, until next time!

HEY, What a Minute, What About Synthetics?

Ahh yes, we’ve all heard the horror stories of mixing or switching between synthetic and conventional oil. Aeration, leaking seals, engine damage… they’re all technically possible. The thing is, it can take some time and neglect for these to become real issues.

It’s important to note that switching between the two at oil changes over the long term should probably be avoided. However, if you’re in a bind and simply need to add some oil, it’s no big deal. Adding synthetic to conventional or vice versa isn’t going to instantly destroy your pride and joy.

Keep in mind that the synthetic oil will not work as well once diluted with conventional. Once again, we’re talking about a quart or two at most when topping off. Often it’s much less than that, and it will be much better having a mixture than simply running the engine while low on oil.


(I’m going to casually ignore the fact that THIS is the internet as well)

There are more tin foil hat theories regarding things such as mixing brands. Some say the chemicals or additives can react badly between different manufacturers, or cancel each other out, or start an intergalactic conquest. 

I say there’s no proof of any of that, and assuming that there may be a “risk” since we “don’t know” how they’ll react is the automotive equivalent of staying indoors for fear of getting hit by a meteorite. SURE it’s possible, but incredibly unlikely. On top of that, if the meteorite made it through the atmosphere, I don’t think your roof is stopping it either.

Similarly, sure, there’s a risk in mixing brands. There’s also a risk in adding oil at all. Maybe the bottle you have got filled with water, or sand, or that liquid glass stuff they used to lock up engines back when “Cash for Clunkers” was a thing. But, I digress.

The point, again, is if you’re low on oil it’s still going to be better HAVING some in the engine than running it low. The chances of a reaction between brands is slim to none. Even if there were one, it’s unlikely that you or the engine would even notice.

Preparation is Key

Before we wrap up I’d like to mention a few good ideas that can prevent you from being in this situation. Firstly, make sure you DO check your oil level regularly. You’d be surprised how quickly a leak or oil consumption issue can pop up. Secondly, take some time to jot down the type and grade of oil your car calls for on a post it note, and stick it to the inside of the glove box. 

Finally, keep a spare quart in the trunk, just in case. Double or triple bag it, and tie a knot with the handles so you won’t have oil everywhere if the bottle somehow cracks open. Doing this has helped me out twice now when I was in the middle of nowhere and needed to top off. 

A little preparedness can go a seriously long way, but it helps having a back up plan. Stick to the guidelines if you can. But remember that if you’re just topping off, pretty much any common automotive engine oil will do.

Happy motoring, friends.


  • Ben

    From his early days playing the original Gran Turismo and with his Hot Wheels car set, Ben has had a long interest in all things automotive. His first foray into the world of automotive journalism was way back in 2009 and since then he has only grown more interested in the industry. Ben also runs and heads up the video production side of Garage Dreams, focusing on small informative documentaries about some of the world's most legendary cars.

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