A transmissionn oil cooler is always a plus. Heat is the #1 enemy of an automatic and towing generates lots of heat, especially hills (whether up hill or descending with the tranny in a lower gear).
Here's a table showing the life of standard tranmission fluid at various temperatures:
> Temperature Approximate Life of Trans. Fluid
> > 175 F. 100,000 miles
> > 195 F. 50,000 miles
> > 212 F. 25,000 miles
> > 235.F. 12,000 miles
> > 255 F. 6,250 miles
> > 275 F. 3,000 miles
> > 295 F. 1,500 miles
> > 315 F. 750 miles
> > 335 F. 325 miles
> > 355 F. 160 miles
Synthetic tranny fluid holds up much better in heat and is a worthwhile investment for any tow vehicle.
And here's an estimate of transmission life taken from a mid-90's Chevrolet service manual:
Trans Temps vs. Life Expentancy in miles
> > ?Farhenheit = Miles
> > 175? = 100,000
> > 195? = 50,000
> > 220? = 20,000
> > 240? = 10,000 Varnish Starts to Form
> > 260? = 5,000 Seals start to harden
> > 295? = 1,500 Plates start to slip
> > 315? = Seals and Clutches Burn up and Carbon Forms