I use Amsoil 75W-90 synthetic & change every year. This is one area I believe synthetic makes sense for even the frugal minded people. Because it only takes a few oz's, buying the 'cheap' stuff will save less than a dollar per change so less than a tank of gas over the life of the bike & since you can safely go longer each change it would be a wash.
I have a table lift and do most of my own maintenance on the Wing. I change the final drive oil every time I put on a new rear tire. This makes it much more assessable and you don't have to worry about the old oil draining onto your tire and / or rim. Works for me...YMMV.
What's the oil capacity of the final drive? Watched two youtube videos. One guy changed the oil and said he only put in about 4 ounces on the refill. Another guy put in over 40 ounces. Both were using the walmart syringe.
Met a long time GL-18 owner and we were talking about the rear diff fluid and how hard or messy it was to replace the oil. He suggested using an oil squirt can. I had one laying around so I cleaned the old oil out and refilled with the gear oil. I had to refill it once, but there was no mess and...
I have a 5 liter bottle of gear oil I bought for my scooter. I am wondering if it is OK to use in the Goldwing. The specification is SAE - 85 W 140 EP, API - GL-5
Looks like the GL-5 is covered, but I am wondering if the viscosity of 140 is too far out?