Tranny fluid doesnt not have the cleaning ability that some claim. Motor oil has better cleaning ability than does transmission fluid. And unless you run the fluid until its leather brown or something, I cannot see it building up varnish and what have you.
Trannyman would be one better to state this, but I have personally never seen a transmission with varnish build up. Now mind you Ive never broke down a tranny either but I have never seen it in the pan or in any components that reside in the pan. Whereas with motor oil you can see varnish build up over time in areas like the valve train.
Just my observations and opinion though.
