Like if u ar just gonna drop the pan & do a fluid & filter service & drain whatever fluid comes out. Then put new back in.

Is it a problem just say you had Shell brand Dexron III Transmission Fluid in ur car & you were to buy Castrol Dexron III transmission fluid is that a problem mixing the brands even tho they are both Dexron III. As long as they are both conventional oil based?

Specifically the transmission i'm wondering about is a 1986 Torqueflite 3spd Automatic out of a K-Car.

 

You should be OK as long as they are both Dex III. I never had a car come back to the shop after I topped off the trans fluid, and used what we had and not knowing what brand was already in the trans.