What Happens If You Mix Different Engine Oils? (Synthetic + Conventional Explained)

What happens if you mix different engine oils?

Have you ever topped up your engine and realized afterward that you used a different type of oil?

It’s a common situation. And no, your engine won’t fail because of it.

But what actually happens when you mix different oils? And is it something you should keep doing?

Let’s clear up the confusion once and for all.

What Happens If You Mix Different Engine Oils

What Happens If You Mix Different Engine Oils?

When you pour two different oils into one engine, a few things happen inside the crankcase:

  1. Additives compete. Every oil brand and type has its own formula, including detergents, anti-wear agents, and friction modifiers. When mixed, they may neutralize or reduce the effectiveness of each other.
  2. Viscosity balance shifts. Mixing a 5W-30 with a 10W-40 changes the thickness and flow characteristics of your oil.
  3. Thermal performance drops. Mixed oils don’t handle heat and oxidation as efficiently, which can accelerate breakdown.
  4. Wear protection weakens. You’ll still have lubrication, but not at the optimized level synthetic oil provides.

In short, it works, but not well. The mix won’t damage your engine immediately, but it won’t deliver consistent protection either.

Mixing Different Brands of Synthetic Oil

Even mixing two full synthetic oils (like AMSOIL and another brand) isn’t ideal. While it won’t cause mechanical failure, every manufacturer uses unique base stocks and additive chemistries. When you blend them, you lose the engineered balance that gives synthetic oil its edge.

That means:

  • Additives might clash or cancel each other out.
  • Foaming or sludge resistance may decrease.
  • Overall performance consistency goes down.

To get the full benefit of synthetic technology, stick with one trusted brand throughout the oil’s service life.

Professionals and performance drivers prefer AMSOIL Signature Series Synthetic Motor Oil, designed for long-lasting protection, extended drain intervals, and stable viscosity in all temperatures.

What If You Mix Different Viscosity Grades?

If you accidentally mix 5W-30 and 10W-40, your car will still run, but it’s not optimal.
Viscosity determines how easily oil flows at different temperatures.

When mixed, the resulting oil can be unpredictable:

  • In cold climates (like Canadian winters): Thicker oil may flow slowly on startup, causing early wear.
  • In high heat or towing conditions (common in the U.S.): Thinner oil may lose film strength under load.

Modern engines rely on precise lubrication. That’s why manufacturers specify exact viscosity grades, and why using a consistent, high-quality synthetic oil keeps everything in balance.

Why Mixing Oils Isn’t Worth the Risk?

Mixing different oils doesn’t cause instant damage. But over time, it compromises performance.

Here’s what you risk:

  • Unbalanced additives
  • Increased oxidation or sludge
  • Reduced cleaning ability
  • Shortened oil life
  • Lower fuel efficiency

Engines are more advanced and tightly engineered than ever. To get the protection they’re built for, consistency matters.

If your goal is smoother performance, fewer oil changes, and a longer engine life, don’t mix. Choose one premium synthetic oil and stay with it.

A Better Option: Use One Premium Full Synthetic Oil

Instead of mixing oils, use a full synthetic that performs across all driving conditions.

AMSOIL is formulated with advanced synthetic base stocks and proprietary additives to deliver consistent protection, no matter the climate or driving style.

Here’s what sets AMSOIL apart:

  • Uniform viscosity and excellent film strength
  • Outstanding protection against wear and deposits
  • Easier cold starts and faster lubrication
  • Exceptional heat resistance and oxidation control
  • Extended drain intervals, up to 20,000 miles or one year

Whether you drive in freezing Canadian winters or hot U.S. summers, AMSOIL keeps your engine protected and running at peak efficiency, without the uncertainty of mixed oils.

FAQs

Can I mix synthetic and conventional oil in an emergency?

Yes, it’s safe short-term. Just replace it with a full synthetic oil at your next change.

Can I top off with synthetic oil if my car uses conventional oil?

You can, but performance will match the weaker oil type.

Can I mix two synthetic oils from different brands?

Yes, but it’s best to avoid it; the additives are balanced for specific formulations.

Will mixing oils void my warranty?

If the final blend meets your manufacturer’s API or ILSAC specifications, your warranty stays valid. But for reliability, it’s better to use a single brand and viscosity.

Conclusion

Mixing different engine oils won’t destroy your engine, but it does reduce performance and protection.

If you care about long-term reliability, better fuel economy, and fewer oil changes, the solution is simple: use one trusted, high-performance synthetic oil.

That’s why more drivers across the U.S. and Canada rely on AMSOIL Synthetic Motor Oils, engineered to outperform, protect longer, and give you peace of mind every time you start your engine.