Lou - I'm going to say NO to BOTH!
1. As far as I know, there's no requirement to periodically drain and fill the hydraulic system. Of course, if the fluid was contaminated or showed discoloration, I'd replace it. My fluid is still the OEM fluid it had in the trailer when I bought it new in 2007.
2. Emphatic NO!!! Do not use Dexron 6 in the HiLo llift system. The original fluid was Dexron II, and Dexron II-V are compatible with the OEM fluid. Dexron 6 has been changed so that it is not acceptable. I THINK it has a completely different viscosity property.
Hope this helps and if anyone has different advice (or wants to expand on this), feel free to add your thoughts.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew