What happens if you accidently mix in engine oil with different viscosity?

I am using 20W50 engine oil and topped it off with a quart of 10W30 not realizing that I had 20W50 in there. Will there be a problem? Thanks. Joe

Im not an oil expert, but I don't think it would do any damage. Im guessing it would be like mixing 91 octane with 87 equally and get 89. So in your case, it will narrow the range of your oil viscosity until the additives wears out. Just my 2 cents.

Just to test it out, pour some 20-50 in a clear glass and then 10-30. Then mix them up. If they separate then they don't dissolve with each other. Either ways, I don't think it would cause any problems as both oils will still be going through and lubricating your engine.

10w-30 + 20w-50 = 15w-40. multi weights are compatible. As a matter of fact, synthetics are compatible with conventional oil.

It's just a matter of time before Road Rage see's this thread, I am sure he will chime in on this.

No worries on mixing. Any particular reason you're using 20W-50 in CA?

Racer is correct - the viscosities sum and then divide by total number of quarts = effective viscosity.

I do not recommend mixing oils from different companies, unless you are adding one quart of syn to a minoil fill to get a synblend. Even then, probably better to mix the same company's min and syn.

Why are you using such a thick oil in your car? 20W/50 may not work well in the F20C due to the very high tolerances and increased wear at startup.

I have always thought that, and have contemplated adding a bit of synthetic to my mineral oil. I know the advantages of synthetic for heat breakdown and as I do lots of driving schools I can see the advantage. However, I have read too often that synthetic smoothes out the VTEC transition, and I really like it not smooth.

Thank you all for the informative posts. I will just leave it until the next change.

Catastrophic oil amalgamation resulting from the anearobic combination of ions and cations in a viscous sludge!











j/k. You'll be fine. Give it lots of warm up time or go back to the proper viscosity.

Hey, that was my line!!

Mixing oils of different weight and brand is not good. It will cause the oil to foam.

I'm a Mechaincal Systems Engineer for Aloha Airlines. In the aviation industry we do not mix oils of different weight, and brand. I would take this same rule holds for car engines too.

But if your low on oil and there is none of the same available, then yeah, you should mix them, but replace as soon as possible.

there should be something on the web about this. I'll check it out.

Here is a site from the military:

I was incorrect: Weight doen't matter, but Brand does.

The oil additives don't always match up. Causing oil foaming, and oil being pushed out.

Don't Mix Oil

I think this is what Road Rage has being saying all along. Thanks for the back up.

Watch it - you are starting to sound like me - I have a copyright on me, which is a wholly owned subsidiary of Myself. I am represented by the law firm of Me, Myself, and I.

Your copyright is U.S. only. Mine is a Canadian one. First one to go international gets it all.