Originally Posted by PeterW
Proper and equal is easy. I have a bamboo skewer in the garage with tape around it I use as a refill marker. Do it properly once, measure and mark it once with the front fully down and the springs poking out.
Next time loosen the top clamp bolts, undo the caps, push the front of the bike down, pump the oil out that you can get at from the top (~2/3) (my forks don't have lower drain holes) and refill with fresh to the mark. Put it back together.
5 minutes every six months is a LOT less effort than several hours every two years, and I'll bet on average my oil is in better condition than yours. It's not engine oil (where this would be a bad idea) it ages totally differently.
You won't ever get all the contaminents out that are sitting on the bottom.
I had a neighbor for several years in San Jose by the name of Jim Lindeman. I talked to him several times about replacing fork oil. He argued very affectively that there is no way to get the proper amount of fork oil in each fork unless you remove the springs and let gravity and piston action drain as much as possible, then remove damper rod and compress forks with hand over open end to use air pressure for force in new fluid. He seemed to know what he was doing.
You bet your fork oil is in better condition than mine? It's not a competition. There is a right way and an easy way to do things. I can, and do, change fork oil in my bikes in about an hour, start to finish, and the experts say this is the correct way to do it. I will spend an hour every year at (least) to know my front suspension is fresh.
of course, I'm nobody to argue with bamboo and tape.