Screwed up tranny fluid and filter change

[Opera House]

Mercon V is a significantly better fluid since it is semi synthetic and it has an improved friction modifier to reduce shudder problems found in some transmissions. It would not cause the problems you have. Chances are you would have had the same problem with Mwecon V. Never seen a Dexron III that wasn't also Mercon. Additives and flushing will not solve this.

 

[caldi55]

Honestly the transmissin runs great its just once it runs for 10min or so it starts to want to downshift on its own or delay the upshift (when accelerating) and you have to let off the gas for about 3 sec. and then it will shift.

 

[caldi55]

I added the friction modifier and now its running great

 

[Rhett]

You're not getting conflicting advice. If you forgot the o-rings or the o-rings did not seat properly when you changed the filter, then you will have air leakage and this can cause strange shifting behavior that you are describing.

As far as the fluid goes, you need to do what aldive, and others said; you need to flush the entire system, and it will take about 15 quarts to do that. You drain a quart or two out thru the cooler, then pour in a quart or two at the filler tube. Repeat.