I wouldn't know, I think the Dex Merc is fine. I have one AW-4 that's fine at nearly 300K. Radi or Cruiser or someone else might have something to say.

 

it's a dex/merc just a synthetic for high mileage vehicles. Thanks for the references

 

i have a aw4 with over 200k on it and have no factory cooler. I have to do a tranny flush n replace pan gasket and rear seal. I was gna do flush with stp dex/,merc then run valvoline high mileage permanently. Does just doin that seem to be enough or should i add a cooler? I live in fl where gets hot, but not gna be doing any off-roading. Was told you're a good person to ask. Thanks

 

It wouldn't be unusual for an AW4 to run 160-180 degrees in normal operation and exceed 220 when it's working hard. They operate on the warm side.
Operation over 180-ish does shorten the life of the fluid and the factory set the service interval accordingly. Every 30K miles (12K for service schedule "B") so the reduction in life is irrelevant.

 

If the fluid is in relatively good condition, (not burned, black, chunky...) I'd drain the trans using the drain plug, then refill. Drive a few days, repeat. Then do it a third time.

I don't care for "flush" products, if that's what you are considering. If there was crap built up in the transmission , I'd rather it was gently dissolved by fresh fluid changes than knocked loose to lodge who-knows-where by a harsh flush product.

There is no harm in adding a trans cooler, but at 200K without one, there may not be a heat problem that needs addressing either. Offroading, towing, definitely.

I've never used the Valvoline high-mileage ATF (max life?) but if it's dex/merc it should be fine.