I would say that there is no Beaver coach that is now running Dextron III transmission fluid that would not benefit from changing over to TranSynd, because the benefits far outweigh any risk of a leak. I have seen a lot of coaches changed to TranSynd, and many of them had 100,000 plus miles and I have never seen one start leaking.
The only question to me is whether it is worth the extra cost. With the extended change interval, and reduced wear and heat, I think that it is a cost effective change.
Gerald