I would think you'd be fine as long as both met the required specs. Years ago I had to add a few qts. of ST Dexron III to Mobil 1 ATF while doing an ATF/filter change on my Buick Century (4T65E) because I only bought 5 qts. of M1.....it ran fine.
Originally Posted by JTK
You do a mix every time you do a drain/fill. If both fluids are compatible with your vehicle you're good to go.
This
I just did a drain and fill on our Explorer that specs Mercon LV, which I was using previously for drain and fills, with Castrol Full Synthetic Multi-Vehicle ATF. It's actually licensed by Ford for use in Mercon LV vehicles. It works just fine. As stated, make sure both fluids are spec'd for your vehicle.
Originally Posted by Whimsey
Originally Posted by JTK
You do a mix every time you do a drain/fill. If both fluids are compatible with your vehicle you're good to go.
This
I just did a drain and fill on our Explorer that specs Mercon LV, which I was using previously for drain and fills, with Castrol Full Synthetic Multi-Vehicle ATF. It's actually licensed by Ford for use in Mercon LV vehicles. It works just fine. As stated, make sure both fluids are spec'd for your vehicle.
In my 04 avalanche tranny, there's a mix of castrol dex6, autozone dex6, max life atf, and a qt of penzoil dex/merc. Original trans and it runs like new. It has a drain plug so I'll do a drain and fill w maxlife when the gallon goes on sale again