When Mixing Is Acceptable
- Emergency top-off: If your engine is low and all you have is a different type (conventional vs. synthetic), top off to safe levels and drive gently until you can get a full oil change.
- Same viscosity grade: If possible, match the manual’s grade (e.g., 0W-20, 5W-30). This helps maintain proper flow on cold starts and protection at operating temperature.
- Correct spec: Prioritize API/ILSAC or OEM specs (e.g., API SP, ILSAC GF-6, dexos) over brand. Spec compliance matters more than labels like “high mileage.”
Why Routine Mixing Isn’t Ideal
- Diluted additives: Each oil’s detergent, anti-wear, and friction-modifier package is engineered to work as a system. Mixing can reduce peak performance.
- Shorter drain interval: A blend of different oils often means you should change the oil sooner than the longest interval advertised by either product.
- Warranty/maintenance plans: If your vehicle requires a specific spec (e.g., dexos1 for many GM gasoline engines), consistently meeting that spec with a single approved oil is safest.
Can You Mix Oil Weights?
Can you mix oil weights? Technically, yes—but avoid it unless necessary.
Mixing, for example, 5W-30 with 10W-30 will likely produce a blended behavior somewhere between them at cold temps, while maintaining similar hot viscosity (the “30”). Mixing 0W-20 with 5W-30 changes both cold-start and operating characteristics, which can affect fuel economy, valve timing performance, and turbocharger protection. Always aim for the exact grade listed on your oil cap or in the owner’s manual.
Best Practices If You Must Mix
- Match the manual first: Use the recommended viscosity and spec whenever possible.
- Top off conservatively: Add the minimum needed to keep the level within the safe range.
- Plan a full oil change soon: Replace the oil and filter with a single, high-quality product that meets your vehicle’s spec.
- Drive gently: Avoid heavy loads and high RPMs until you’ve done a proper change.
What Not to Mix
- Gasoline vs. diesel formulations: Don’t substitute diesel-engine oil in a gasoline engine (or vice versa) unless the oil explicitly lists the required gasoline spec.
- European low-SAPS oils: Some emissions-system-friendly oils for European vehicles have unique specs; don’t mix with non-approved oils.
- Motorcycle/ATV or small-engine oils: These often have friction properties tuned for wet clutches or air-cooled designs—keep them separate from passenger car oils.
Signs You Need a Proper Oil Change Now
- Oil level keeps dropping between changes
- Noisy start-ups, ticking lifters, or increased engine vibration
- Check Engine or oil pressure warning lights
- Oil appears very dark, gritty, or smells burnt
Bottom Line
Can you mix synthetic and regular oil? Yes—briefly, for top-offs or an emergency. For long-term protection, stick to one oil that meets your vehicle’s exact grade and specification, and change it on schedule with a quality filter.
Schedule an Oil Change at Paul’s Auto Repair
Need help picking the right oil—or did you just mix oils and want a fresh start? The ASE-certified technicians at Paul’s Auto Repair will install the correct synthetic or conventional oil and filter for your vehicle, reset reminders, and perform a complimentary inspection. Call Paul’s Auto Repair or book your oil change online today to protect your engine and keep your maintenance on track.
