Well, I can tell you from having to re-seal side covers on transfer units, old gear oil does get nasty, leaves filthy staining inside.. so I would advise periodic change of the oil to be safe, as clean oil will not eat through the seals so fast.. You can get MOBIL-1 synthetic gear oil for maybe $10/qt.

Now a curve ball, this is for real, as I did the work...

Santa Fe 4WD shows up on hook, customer states rear end locked up,, wont roll... I get in, rear is bound up, but we get it to break free, though extremely noisey and chunky.. insides blown and differential case has nice crack, left axle out of place... replaced rear differential assembly under warranty.....

After install, I back the vehicle off rack, and start to roll down the aisle, there is a rolling bearing on steroids sound.. across lot, I know what that is.

Put car back up in air, the transfer is trashed,,, removed the fill plug, black, and stinks into next month, removed drain plug, NO OIL !! It all leak out somewhere.

I was laughing and waiting to hear what would become of a warranty claim for a transfer and differential claim on a '01 or '02 with 98,000 mile, that was at least $3000 claim I think...