Can You Mix Different Brands of Motor Oil?
Yes, mixing different brands of motor oil is safe if they're the same type (both conventional or both synthetic) and the same viscosity (e.g., both 5W-30). All motor oils sold in the US must meet API standards and are designed to be compatible. However, mixing different viscosities or types (synthetic with conventional) isn't recommended.
Key Takeaways
- Motor oils meeting API (American Petroleum Institute) standards are formulated to be compatible with each other.
- In an emergency, any oil is better than running low—add whatever's available and change it soon.
- High-mileage oils have different additives that may not mix ideally with regular oils.
Explanation
Motor oils meeting API (American Petroleum Institute) standards are formulated to be compatible with each other. The base oils and additives are similar enough that mixing brands won't cause chemical reactions, separation, or damage. This is why you can safely top off between oil changes with whatever's available.
Viscosity (the weight, like 5W-30) should match. Mixing different viscosities creates an unpredictable blend that may not protect properly at all temperatures. If your car needs 5W-30, adding 10W-40 changes the oil's flow characteristics in ways the engine isn't designed for.
Synthetic and conventional oils are technically mixable—synthetic blends are literally this. But intentionally mixing them means you lose the full benefits of synthetic oil (better protection, longer life) while still paying partially for it. Pick one type and stick with it.
The viscosity number tells you two things. The first number before the 'W' (which stands for Winter) indicates cold-flow performance: 0W flows best in extreme cold (-40°F), while 10W is adequate for moderate winters. The second number indicates viscosity at operating temperature (212°F): 30 is thinner, 40 is thicker. A 5W-30 oil flows well in cold starts and provides moderate protection at operating temperature. Most modern cars use 0W-20 or 5W-30 because tighter engine tolerances benefit from thinner oils that reduce friction and improve fuel economy.
Oil change intervals have extended significantly with modern oils and engines. The old 3,000-mile rule was appropriate for conventional oil in older engines but is outdated for most vehicles today. Synthetic oil typically lasts 7,500-10,000 miles or more, and some manufacturers like BMW and Toyota specify 10,000-15,000-mile intervals. Your owner's manual provides the correct interval. Many newer cars have oil life monitoring systems that calculate when a change is needed based on driving conditions, engine temperature, and mileage rather than using a fixed schedule.
Things to Know
- In an emergency, any oil is better than running low—add whatever's available and change it soon.
- High-mileage oils have different additives that may not mix ideally with regular oils.
- Some European cars specify particular oil certifications; brand doesn't matter, but certification does.
- Diesel engine oils and gas engine oils should not be mixed.
- Look for the API donut symbol on the bottle. The current gasoline engine standard is SP (introduced 2020), which is backward-compatible with older SN, SM, and SL ratings.