Can I mix 5W20 and 5W30 oil?

Question

Clark · Accepted Answer

You can physically mix 5W20 and 5W30 motor oils in an engine, but it is not recommended and should be avoided whenever possible. Blending them creates a final oil viscosity that is neither 5W20 nor 5W30, which can compromise the engine protection and performance specifications set by your vehicle's manufacturer.

The key risk is altering the oil's High-Temperature High-Shear (HTHS) viscosity, which is critical for maintaining a protective lubricant film between moving parts at operating temperature. 5W30 oil is specifically formulated to be thicker than 5W20 at high engine temperatures. Mixing them results in an unpredictable, intermediate viscosity that may be too thin for an engine designed for 5W30, leading to increased wear, or too thick for one designed for 5W20, potentially reducing fuel efficiency and increasing engine load.

Manufacturers specify an oil viscosity grade after extensive engineering validation to ensure optimal protection, fuel economy, and emissions system performance. Using a non-specified viscosity, even as a one-time mix, falls outside these tested parameters. For instance, industry data from lubrication engineers indicates that using an oil even one grade out of specification can increase engine wear rates under severe operating conditions.

If you are in an emergency situation with no other option—for example, your oil level is critically low and only a different viscosity is available—topping off with a quart (approximately 1 liter) to safely drive to a service location is considered a short-term contingency. The priority should be to perform a complete oil and filter change with the correct specification oil at the earliest opportunity.

For clear reference, the table below outlines the primary distinction between these two common grades:

Viscosity Grade	Low-Temperature "W" Rating (Winter)	High-Temperature Viscosity (at 100°C)	Typical Application & Characteristic
5W-20	Same cold-start performance (5W)	Thinner when hot. Lower internal friction.	Specified for many modern engines to maximize fuel economy.
5W-30	Same cold-start performance (5W)	Thicker when hot. Higher film strength.	Common for wider operating conditions, older engines, or performance use.

The "5W" prefix is identical, meaning both oils have similar flow characteristics in cold weather. The difference lies entirely in the number after the "W," which defines the oil's thickness at the engine's normal operating temperature. The Society of Automotive Engineers (SAE) J300 standard governs these classifications, and a higher number denotes a higher viscosity. Ultimately, for long-term engine health and performance, consistently using the oil weight listed in your owner's manual is the only recommended practice.

DelElaina · Answer

As a mechanic, I've seen this question pop up in my shop more times than I can count. Here's my straight talk: Yes, you can mix them in a pinch to avoid running the engine dry. I've done it for customers stranded on the highway. But listen—it's strictly a "get you home" or "get you to the shop" move. Drive gently, avoid high RPMs, and plan to get that mixed cocktail drained out and replaced with the proper oil ASAP. Your engine's tolerances are designed for a specific thickness; guessing with a mix is like wearing shoes that are almost your size. They might work, but not well, and not for long.

Mario · Answer

From an engineering perspective, the primary concern is the dilution of additive packages and the deviation from the designed hydrodynamic lubrication regime. Motor oils are precisely balanced formulations. Mixing two different viscosities disrupts the carefully calibrated viscosity index improvers and other additives.

This creates a fluid with unknown shear stability and thermal characteristics. In an engine designed for 5W20, a thicker mixed oil increases parasitic drag on components like the crankshaft, reducing fuel economy. Conversely, in a 5W30 engine, a thinner mix may not maintain sufficient oil film thickness at high temperatures and loads, potentially leading to boundary lubrication and accelerated camshaft or bearing wear. The risk is probabilistic and may not cause immediate failure, but it introduces an unnecessary variable that voids the manufacturer's validation.

CollinLee · Answer

I made this mistake once. My car takes 5W20, and I was a quart low. All I had in the garage was leftover 5W30 from my old truck. I added it, thinking "oil is oil." The car ran, but I immediately noticed the engine felt a bit more sluggish, and my gas mileage dipped noticeably over that next week. It wasn't a huge difference, but it was there. It felt like the engine was working harder. I didn't wait for the regular oil change interval—I had it changed out after about 200 miles just for peace of mind. I learned my lesson: now I keep a sealed bottle of the right oil in my trunk for emergencies.

Caden · Answer

Consider these practical points if you're contemplating mixing viscosities.

This is most acceptable only as a temporary, limited-volume top-off. Adding one quart of a different viscosity to a sump that holds 5-6 quarts has a lesser impact than mixing half-and-half. The effect is proportional.

Check your vehicle's warranty terms. Using an oil that does not meet the exact manufacturer specification, which includes viscosity grade, could potentially be used to deny a warranty claim related to engine damage. It's a rare scenario, but a real risk.

For older, high-mileage engines with established wear, some technicians observe that a slightly thicker oil (like moving from 5W20 to 5W30) can reduce oil consumption and noise. However, this should be a deliberate, permanent switch after research, not an accidental result of mixing. The correct procedure is to drain the old oil completely and fill with the new, thicker grade.

If you regularly need to top off oil between changes, you have a consumption or leak issue that should be diagnosed. Relying on mixing oils is not a solution for an underlying mechanical problem.