A sealed container Vs open to the atmosphere and temperature changes can gradually weaken the lubrication properties of a fluid. Typically 30K miles or 5 years is a good interval to replace diff and transfer fluids. The P/S fluid pickup tiny rubber particles in hoses and becomes a sand blaster on all other soft parts causing unwanted wear. I replace P/S fluid once every 2-3 years and the P/S system in all my vehicles (except the cruiser) are still factory! Even my Prizm at 288K miles does not lose fluid. Cruiser got a Reman rack by the previous owner.I’m somewhat confused as to why fluids and filters would break down sitting in a garage any more than they would sitting on a shelf unused.Tires..maybe.But other rubber components,not so much.
Also grease the drive shafts.
The best gear oil in the market is Redline and is bit expensive ( I use Redline gear oil in my Prizm's manual transmission and it made a MASSIVE changein gear shifts). I have Valvoline gear oil in the diffs/transfers and I am happy with the performance.