It's theoretically fine - all the manufacturers that meet a certain spec (listed on the bottle) are certified as being interoperable.

In the real world people have mixed results (pun intended). I'd probably mix them as you intend, but I'm very much an atheist in the motor oil religious wars.

KeS

 

don't do it, it might explode!

I usually just dump my old oil on top of that 1/2 quart leftover. Unless I'm doing my wifes car at the same time, it takes 5.5 quarts.
So 2 5 quart jugs is perfect to do both.

Anyone else notice that Autozone sells Mobil1 in a 1 gallon jug, and Walmart in a 5 quart jug?

 

There's something to say for consistency. If it dipstick starts looking like a rainbow, maybe it's time to stick to one oil.

 

Use the remaining oil for oil changes on your neighbor's mower (the one who doesn't have a clue about maintenance).

I have two of those types of neighbors!

Then, enjoy the beer they give you and move on.

:act002:

 

Only thang that makes me think not is I know Penzoil is synonymous with detergent oils so I am leary about Penzoil. In the 70's Penzoil was considered the worst thing you could do to an engine, now, it sounds like they have pretty good syntetic oil from what I read on the interwebben, and everything on there is the truth right?:act035:

 

So long as they're all the same viscosity (which you state they're all 5w-20), then it's perfectly fine. You'll do no harm. No different than if you found yourself a quart low between changes and happened to add a different brand.