A little of both. Should do it every couple of years or about every 6,000 miles.
really? so in the first 6 mts i had my bike i would change it 3 times? i have seen some stuff online saying to do it yearly on street bikes and after every weekend of track time. I would hate to buy that much fork oil. BUT i did do mine after 18k miles... a bit over a year of me having the bike.... but it was now 4 years old(05) all i can say is HOLY SHIT!!! the oil was water thin and had lots of crude in it.
see the stuff that settled to the bottom of the bucket...
and wow i didnt know the forked were that dead... and 2 mts before i had the supen setup by a supen shop... they said the rear was gone but the front was ok.
do yourself a favor people and change your oil. Im trying to get a friend to bring his 07 over and lets do it but he likes to drag ass and whine. he also needs to do the valves@ almost 30k on the motor and thats the same deal.... drag ass and wont order a v.c. gasket or new plugs even when the bike runs like ass. oh well
I fail to see the problem with my suggestion. If you put 6,000 miles on your bike in three days, it's still 6,000 miles. Dave moss is the MAN that told me to fill with 5 wt. oil. I figure the average rider places about 3,000 miles or so a year. If you do more than that, then at 6,000 miles you should plan on it. If that is 3 months, a year, or 2 years, you get the picture; I hope anyway......
A normal, average street rider will not need to worry about it as much as a regular track goer. Those of us like me, that only ride the track will plan on doing it at least once a year. I do about 200 miles a track day and do about 12 track days a year. That works out to 2,400 miles a year or so. The only reason that serious track riders need that much attention on it is because you want EVERYTHING in tip top order for best performance. An average JOE on the street will not suffer as much from the loss in performance from less than perfect suspension.
The stock oil in the forks is WATER THIN. I believe the stock stuff is literally 0 wt. Basically enough to provide resistance and lubrication. It gets dirty like that for more than one reason. There is some electrolysis happening, as well as oxidation from the aluminum and steel parts. If the bike were to sit and not do anything there would still be dirty colored oil after a while. The performance of the shock, or fork, is not completely destroyed by this happening. It does what it needs to pretty well. It just does it better with fresh oil.
In the end it comes down to best practices and procedures. Once a year is best for an average guy, however it is more expensive and no real performance is seen. Every 6,000 miles is a good suggestion, as it will be often enough to be good and not fiscally burdening. If you ride more and place over 6,000 miles a year, then you should at least plan on it, although it is not REQUIRED. If you NEVER did it, I would be hard pressed to say you would ever notice; assuming you were an average street rider. Where it changes to something of real value is when you start riding the track often and for performance results.