The 90 wt gear oil should be changed at least once a year. I change mine before the riding season begins. Transmission, driveshaft, & rear end. I run the bike to get everything warmed up. Then I put pans under each drain and let it drain overnight. I ordered my 1977 R100/7 from the factory in November 1976, picked it up Mar1977. I have ridden “Fred” 189,000 miles before pickling it in 1999. It is stored in a Railroad boxcar in Colorado. Once a year I pull the spark plugs and squirt some 10W30 oil in. Then use the kick starter to turn over the engine. I have 5 bikes currently, 1977BMW, 1981 Suzuki, 1983 Kawasaki, 1984 HD, 2016 HD