There supposed to be a way to drain it? There's a fill plug but no drain that I can find. Maintenance charts shows to check fill every 250hrs. But nothing about changing it out. Oil in mine is white froth looking. I sucked out what I could but it's not much as the shaft is in the way.
I should mention, S250, but many models used this right angle drive.
Rented an S300 once to move heavy wet spring snow…very impressive machine! Not sure about draining fan gearbox though. Our 773 did move an impressive amount of air; never did any service though on the fan drive while we owned it.