I use any good quality gear oil and change it every season.
From what i have read, it seems that lots of people are sucking out the oil with good success.
Personally, i prefer to remove the cover disassemble most of it (within reason)and clean out all the shavings and crap that the magnets catch. Check out my chain and gears and Give everything a good looking over and reassemble
Im sure its overkill but knowing i did the best service i could gives me peace of mind and thats important to me.
I change out all driveline bearings every other year, and regrease them between changes. Again, i know its overkill
But overkill is a great thing when your ocd. Lol