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When should I replace fork oil?
Originally posted by @supamodel
Although people are hesitant to miss an engine service on motorbikes, there's one area that generally receives little attention: fork oil.
Most people do it whenever their fork seals leak and that's about it. Most manufacturers don't even specify a fork oil change interval; Ducati do specify 30,000 km on my Monster and I use that as a rough guide. I don't want to go further than that without changing the fork oil.
I had a leaking fork seal on a Ninja 250R and needed to fix it, which involves dismantling the forks and putting new oil in. Kawasaki don't tell you anything about when you might want to redo the fork oil. For reference, this bike has done ~26 000 km. In the attached pic, right is old fork oil and left is new.
Although it's very hard to photograph, the old fork oil is all sparkly from metal particles in it. Those metal particles act as a liquid form of sandpaper and enlarge all the holes within the fork (the valving) which then changes how the suspension works. Not good! Best option is to get your fork oil done often to avoid any problems.
Once I have enough pictures collected I'll do a guide to how to change fork oil, though there are other good guides out there and, as always, the workshop manual for your bike is a great place to start.