After buying my new used motorcycle, I noticed it wasn’t maintained very well by whoever owned it previously. There aren’t a lot of miles on the odometer, so no real damage has happened. It’s always best to nip it in the bud and change out the vital fluids when you can!
After looking up the gear oil needed (80w-90), I looked at the rear drum and it’s super simple. A drain plug, a fill plug, and a axle plug in the center that isn’t a part of any of this. I also found out from reading that the bike is designed to have the gear oil changed while the bike is on it’s kickstand, which makes life easy.
I purchased a bottle of Royal Purple 75w-90 hypoid gear oil and an injector syringe from Amazon.
I first put my oil catch underneath of the rear drum on the bike, then I pulled out a 17mm socket and loosened the large fill plug on the middle right of the drum face. Check for debris from the oil on it, and wipe it down. If there’s metal debris, there may be issues inside there so keep your eyes open. Place the plug aside, then down to the drain plug which required a significant more amount of force to open. (it was really too tight, the person who did it before over-torqued it) Once it was loosened, I manually unscrewed and removed the drain plug and let the oil drain for a minute or so as I did other things. Mine did not have a crush washer, and I did not have any available to me at the time. When I came back, I wiped the plug off, and screwed it back on. I’m lacking a torque wrench but I’ve done bolts through the years enough to know when it’s good enough. The recommendation is 14 ft/lbs of torque. I got my trusty syringe out that I bought for this occasion! The amount needed to fill the rear is 4.1 oz, which is about 120cc. I filled my syringe to 60cc, and since I didn’t have a long hose to go deep into the rear drum, I injected the oil with extreme force. (there’s a deep hole between the outside and the inside in the fill plug) I then re-filled the syringe with another 60cc of gear oil, and force injected it into the rear drum. Once that was done, I wiped clean the fill plug bolt rear seal and traced a small amount of new gear oil over it with my finger. I screwed it back onto the thread of the fill plug hole steadily by hand to make sure there’s no cross-threading, then socket-tightened it. It needs very little tightening, be gentle with it as it only needs 9 ft/lb torque.
Once you’re done with that, wipe everything down and clean up. It’s time to warm that rear gear up and make sure everything’s in good shape. I ran her about 5 or so miles at 45-50 MPH and everything went fine. Actually, smoother since the gear oil that was in there previously was black as sin and dirty. You won’t need to do this but every 30-40K miles, but I think I’ll be doing it once a year since it’s so damned easy and cheap.
The Sociopathic Mind of a Certified Eclectic 
Wish you had a video on changing the gear oil
Unfortunately I no longer have the bike since I traded it in for my current bike which has a belt drive, but it’s pretty straight forward once you’re down there.