Can motor oils of different brands be mixed?

[VanAniyah]

Motor oils of different brands can be mixed, but it is not recommended. If it's for emergency purposes, it might be acceptable. It's important to note that when mixing different oils, the viscosity grade and performance level must be the same, for example, both should be fully synthetic 5W30 or fully synthetic 5W40. The potential impacts of mixing different brand motor oils are: 1. Turbidity: Most modern motor oils use sulfonates as dispersants and detergents, while some use salicylates. These two types are not compatible, and mixing them can cause layering, turbidity, and reduced cleaning performance; 2. Abnormal exhaust: Mixing different brand oils may lead to abnormal exhaust smoke, such as black or blue smoke. The mixed oil might become diluted, allowing it to enter the combustion chamber and produce blue smoke, or it might cause poor cylinder sealing, resulting in black smoke; 3. Sludge formation: Mixing different oils can easily lead to sludge formation, which reduces the cooling effect of the lubricant, causing the engine to overheat and potentially leading to malfunctions; 4. Accelerated wear: The anti-wear properties of the mixed oil can change significantly, potentially breaking down the oil film and causing wear between the piston and cylinder wall, or even piston ring breakage in severe cases.

[VanNicole]

When I first started repairing cars, I was also curious about this issue. But after taking apart the engine, everything became clear. The additive formulas of different brands of engine oil vary too much, and mixing them can produce flocculent precipitates that clog the oil passages like an old sewer. Last year, I worked on a BMW that had mixed oils, and the filter was clogged with coffee-ground-like hard chunks. The overhaul cost the owner over 20,000 yuan. Mixing mineral oil with full synthetic oil can also cause layering, reducing lubrication effectiveness by half. The worst case I've seen was a Civic that suffered cylinder scoring after mixing oils, with deep grooves worn into the piston sidewalls. In an emergency, you can add up to 100ml of the same grade oil at most, but you must completely drain and clean the system afterward. During maintenance, make sure the mechanic doesn’t use leftover no-name oil from the oil gun—it really ruins the car.

[WyattDella]

Driving trucks long-distance for 20 years, I've learned the hard way about mixing engine oils. Once at a service station in Gansu, only Shell was available, so I reluctantly mixed it with the original Castrol in my engine. After just 300 km, the engine sounded like a tractor. Upon disassembly, we found the bearing shells were scratched. An old mechanic explained that anti-wear additives in different oils can conflict—mixing brands may neutralize additives or produce acidic compounds. Now I always keep a small bottle of the original factory oil for emergencies, clearly labeled with brand and specs before adding. Veteran drivers advise: even if the dipstick hits the lower limit, never randomly mix oils—better to call a tow truck. Saving a few dozen bucks on oil isn't worth an engine repair that costs three cases of new oil.

[VonLorenzo]

As someone with a chemistry background, I can tell you that engine oil is a precisely formulated solution system. Just like how mixing skincare products can cause pilling, blending different brands of motor oil may lead to additive precipitation. A classic example is when detergents encounter unfamiliar dispersants, resulting in flocculent sedimentation. Mixing fully synthetic PAO base oil with mineral oil can cause separation, akin to how water and oil don’t mix. In emergencies, if you must mix oils, choose the same grade and specification—for instance, both 5W-30 fully synthetic—but remember to change the oil earlier than scheduled. Long-term mixed usage can cause sludge buildup. I’ve seen Volkswagen EA888 engines with clogged variable valve systems due to mixed oils, where the repair costs were enough to replace two sets of tires.

[TaylorDella]

Here are four key points in simple terms: Daily maintenance strictly prohibits mixing oils, as additive conflicts between brands can lead to oil film failure. In emergencies, you can mix oils of the same viscosity grade, such as both being 5W-30. After mixing, you must replace the oil within 800 kilometers and remember to clean the oil pan. Don’t throw away leftover oil from maintenance; seal the bottle and keep it in the trunk for emergencies. Last year during a road trip, the oil warning light came on on the highway, and I added half a bottle of a different brand to make it to the service area. The mechanic said it was lucky I acted in time, or the camshaft would have been ruined. Now, I always keep a small bottle of original oil in the car, the kind with a sealing ring on the cap is the most reliable.