WhatTypeOfOil.com

Can You Mix 5W-20 and 5W-30 Motor Oil?

You pull the dipstick at a gas station, notice your engine oil is low, but the shelf only has 5W-30 when your car specifies 5W-20. Is it safe to mix them? Here is what happens mechanically.

⚡ The Quick Answer

Yes, you can safely mix 5W-20 and 5W-30. Both oils share the exact same 5W winter rating for cold starts. Mixing 1 quart of 5W-30 into 4 quarts of 5W-20 creates a slightly thicker mixture (approx. 5W-22) that will safely protect your engine until your next scheduled oil change.

Understanding Oil Viscosity Numbers (5W-20 vs 5W-30)

Motor oil viscosity ratings are split into two numbers established by the Society of Automotive Engineers (SAE):

  • 5W (Winter Rating): The number before the "W" measures oil flow at cold startup temperatures (-30°C). Because both 5W-20 and 5W-30 are 5W oils, they provide identical engine protection during cold morning startups.
  • 20 vs 30 (Operating Temp Rating): The second number measures oil thickness at normal engine operating temperature (100°C / 212°F). 5W-30 is slightly thicker at operating temp than 5W-20.

What Happens When You Mix Different Viscosities?

When you blend two different viscosity oils, the resulting mixture sits proportionally between the two viscosities. For example:

Example Viscosity Blends:
  • 50% 5W-20 + 50% 5W-30 = Approx. 5W-25 viscosity
  • 75% 5W-20 + 25% 5W-30 = Approx. 5W-22 viscosity

The engine will experience zero mechanical damage from running an intermediate viscosity. Running slightly low on oil is infinitely more damaging than mixing viscosity grades.

Can You Mix Synthetic and Conventional Motor Oil?

Yes. Modern motor oils are chemically compatible regardless of whether they are full synthetic, synthetic blend, or conventional mineral oil. In fact, "Synthetic Blend" oil sold in stores is simply a factory mixture of synthetic and conventional base stocks.

Can You Mix Different Brands of Motor Oil?

Yes. All engine oils sold in North America must meet API (American Petroleum Institute) certification standards (such as API SP or ILSAC GF-6). API certification requires that additive packages remain fully miscible (compatible) with competing brands. You can mix Mobil 1, Pennzoil, Castrol, Valvoline, or store brands without chemical breakdown.

Want to check the exact factory oil spec for your car?

Look up exact factory viscosity, quarts needed with filter, and oil drain plug torque specs.

Look Up Your Vehicle Oil Spec →

Frequently Asked Questions

Can you mix 0W-20 and 5W-20 oil?

Yes. Mixing 0W-20 and 5W-20 is safe. In cold climates, 0W-20 flows slightly faster at sub-zero temperatures. At operating temperature (20 weight), both oils have identical thickness.

What happens if you use 5W-30 instead of 5W-20 permanently?

Running 5W-30 in a vehicle spec'd for 5W-20 provides slightly thicker film protection at high temperatures. Fuel economy may decrease by ~0.5%, but engine wear will not increase.