With almost 8000 km on the WR250R and a never-ending winter outside, it was a good time to inspect the fork oil. I had no idea what to expect, given I ride about 50-50 dirt/road and most of the dirt around here has a lot of fine, sticky clay in it. My KLR650 benefited from a fork oil change about every 8000 km after a slightly more aggressively dirt-oriented style of riding. For comparison, the RockShox, Marzocchi, and Fox shocks that I've ridden on my mountain bikes over the years all seemed to benefit from at least an annual fork oil change to remain supply--although the newer Fox suspension I ride now seems impervious to trail grime.
An online search for info on the WR fork oil gave ambiguous results. The shop manual only indicates Yamaha fork oil (no weight), yet the dealer where I bought the WR had no idea what oil that would be and said they use Belray anyway. A couple of litres of 5-weight Honda fork oil, and I was on my way. Yeah, yeah--sacrilege not using Yamalube. But we're talking a Showa fork, same as on a Honda.
The shop manual indicates that each fork, when completely drained, requires 613 mL to refill. Disassembling the fork is a piece of cake (loosen the pinch bolts, slide out the fork assembly, and unscrew the top cap. Don't worry--nothing goes "sproing"). The oil in both sides turned out to be nearly perfectly clean. After recycling one fork's worth of oil, I decided to save the other fork's oil for reuse later if necessary.
The Honda 5 Wt oil was like water compared to the OE oil, which I'd estimate was about a 10-15 Wt. One guy's WR blog said he switched to 5 Wt after getting his forks revalved and tuned--and the result was amazing. Now, eliminating the obvious advantages of revalving, I was curious how just changing the oil weight would feel and figured it was worth a try. For reference, I weigh 190 lbs and carry about 25 pounds of gear and clothing, and I tighter up the compression/rebound settings to compensate for the lighter oil.
An initial test ride felt pretty good on our winter potholed roads. Front wheel seemed to track a lot better over the little stutter bumps. Obviously this is hardly a scientific test and warmer temps (i.e. at least above 20C versus the 4C during my test) could make a significant difference. The lesson is, it wasn't a disaster; on the contrary, my seat-of-the-pants impression was quite encouraging.
More to report as testing continues.
Update August 25, 2014: Hmmm, not sure I'm liking the 5 wt. May change it for 10, something with a little more damping so my fork doesn't bounce around so much on washboard roads. This should also improve low-speed damping.