I'm pretty sure I already know the answer to this, but I wanted to consult the experts just in case it's not as bad of an idea as I thought. Anyway, I'm currently using Mercon-V Mobil1 Synthetic ATF and I picked up some Dexron-III Mobil1 Synthetic from Matty Mooling's sale thread a little while back thinking it was the exact same kind of fluid.

Well I recently purchased an Auto Tranny Cooler and was hoping to install it when I did my second tranny service cuz I know I'll have to drain all the fluid to install it anyway. So I went to buy 2 more quarts of the tranny fluid to make up for the tranny cooler and noticed that the fluid I got at Advance Auto is M-V, whereas the fluid I got from Matty was D-III. Would it be a bad idea to mix the two together, or as they practically the same stuff?

 

Alright I'll shorten this up a bit for those that may be too lazy to read..

Is it a bad idea to mix Dexron-V and Mercon-III Auto Tranny Fluid?!?

 

It won't kill anything. You'll be fine.

It's practically the same stuff, the Merc 5 is a bit better, however.