I've got a couple of quarts each of Redline 5w/40 and Eneos 5w/40. I'd like to use both together in the next oil change on my 2000 Legacy.

What are your opinions on mixing them? They're both high end synthetics.

Not really supposed to do that, each has their own type of detergents, and they don't usually take to mixing well. They can fight each other.

 

The oil part would be okay, it's the additives and what they are made up of that don't mix.

I have never heard of any negative effects of mixing different brands of oils personally. I would say you are probably ok unless you plan on racing or trying to run extended intervals with the oil.

Do it! Don't give it a second thought. The anti-wear agents used in motor oil are basically minerals. Boron, magnesium, etc. No ill effects mixing different minerals. But basically they all use the same ingredients. Look at the petroleum quality institute website. Most of the ingredients are the same.