Understanding the dangers of mixing mineral and synthetic motor oils.

The dangers of mixing mineral and synthetic motor oils depend on the types of synthetic bases that were mixed with mineral bases. Polyalphaolefin and diester synthetic bases can be mixed with mineral oil bases, which is done regularly to create “blend” products.

Remember, modern engine lubricants are sophisticated products, formulated to meet the demanding lubrication requirements of modern engines.
The old saying, “oil is oil” no longer applies. Mixing lubricants is fraught with danger — to your car and to your wallet. When in doubt, don’t mix different lubricants. If it occurs accidentally, address the problem immediately.

Why are there dangers to mixing mineral and synthetic motor oils?

"Regular engine oils" are mineral-based products refined from crude oil taken from the ground. Over the past 20 years these lubricants have been refined even further, particularly in the area of viscosity enhancers. This means modern oils flow better over a range of temperatures. This, in combination with engines that sport tighter clearances and better machining, allow for the use of thin oils that both reduce friction and improve fuel efficiency.

For instance, in the world of racing, very few teams are going to be using motor oil with single rated viscosity. Racers not only want efficient operation and greater power, they want the best lubrication of engine parts as quickly as possible. (Start-ups deliver high engine wear, so you want an oil that gets to work quickly.)

"Synthetic oils", which have been around since the 1940s, can have the same natural ingredients as "regular oils" but they are “super distilled” or man made in a chemical plant. Try wrapping your head around the concept of “synthesized-hydrocarbon molecular chains” and base fluids including “polyalphaolefin, synthetic esters, and alkylated aromatics.”

In its mildest form, the dangers of mixing mineral and synthetic motor oils may lead to a degradation of lubricant performance. Mixing the same API grades of synthetic passenger car motor oil and mineral oil-based engine oil won’t damage the engine, but you will lose the performance features you expect from the synthetic.


Polyalkylene glycol (PAG) bases should not be mixed with any of the others unless specialized barrier fluids are used to minimize the incompatibility. When PAGs are mixed into other lubricants, you typically will get strong negative reactions (producing sludge and tacky residue) that require extra effort to flush, clean and correct.

Even if the base oils are compatible, the dangers of mixing mineral and synthetic motor oils is that the additives used to create necessary performance properties could conflict, producing lost lubricant effectiveness.

How to overcome the dangers of mixing mineral and synthetic motor oils.

If you’re not sure of the dangers of mixing mineral and synthetic motor oils, don’t be afraid to call on the experts, such as the professionals at Habot Oil. Your response to a situation where different lubricants are mixed will depend on the products in the mixture, the end-use application, the relative concentrations of products and the total volume involved.