Had my plug in since 03. Bought bike new. Its got whatever the manufacturer put in it. My freinds have WR450 and they HAVE to change every 4 or so rides or else they fail half way through a ride and I can give them heaps of "Yamaha is crap" talk while they change it. I'm thinking about changing it when the bike turns ten years old. It starts nice cold, hot, warm so Why would you change it. It would have a bucket load of hours on the bike, say 350-500 hours at a guess. Bike is a standard CRF 450, no mods.
Mine has always gone grey too in tranny. I change engine oil and tranny oil every second ride religously. I have a freind that races and he changes every ride. He buys cheap oil in 20 Litre containers and says "I change it every ride 4-5 hours, so its not in there long enough to break down". I think it might be sucking up water somewhere though, I have heard to re route the breather pipes into the air box as they suck in water when the gearbox gets hot???? Not sure if this is true, I will try it and let you know. I will also pull out the water pump shaft and sealand check it out this week. I will also check out my bro's 07 crf 450 and see if his oil goes grey. I know he doesn't change it that regularly, however he pits around pretty slowly.