Fork oil gets contaminated from moisture and from the seals moving over the chrome when they have bug guts on them. Handling is noticeable better after changing fork oil after a season.
I think you can change the transmission and final drive oil with one quart of oil. Want to keep that oil clean. As mentioned, being un-syncronized probably means particles develop more than in something syncronized.
Brake fluid absorbs 1-2% of moisture yearly. I would flush motorcycle brake fluid yearly or maybe every other year. Keeps master cylinders happy. I don't know if it's an issue with bikes but with cars, excessive moisture results in a fading feeling when braking.
Any "lifetime" fluid in a car is bs. Like Ford Econolines had "lifetime" differential oil but one for our family business failed at 250k miles or so. If that oil were changed, say every 50k miles, I seriously doubt it would have failed. I change ALL fluids in my Subaru regularly...pump out the master cylinder, refill then bleed the brakes, change coolant, change transmission and rear diff fluid every 30k, exchange power steering fluid every other engine oil change. Things like ABS units and power steering pumps and racks wouldn't ever failed if they had clean fluid. Over 200k miles on my Subaru now (bought new) and haven't had to replace much.