Steve Hoelscher wrote:
If you use a quality, name brand fluid, change it regularly, an additive is not necessary. Lubgard is good stuff and is always a good idea but far from necessary. Changing the fluid regularly is the best preventative maintenance you can do. Note that all Lubgard does is provide a shot of additives. All ATF is is hydraulic fluid with a complex additive package intended to stabilize the oil under heat and load and provide for smooth clutch engagement.
I asked Steve this very thing 10 years ago, followed his advice, and just recently sold my tow vehicle motorhome with over 200K miles. Still running strong and never a tranny problem.