Hence, having a fluid with a high viscosity index helps tremendously because the oil does not break down nearly as fast. Although forks have large surface areas inside of them to contaminate the fluid with aluminum and Teflon (internal sliders) particles. Just sayin, look at centrastokes (thickness) and viscosity index(maintaining ability) when choosing a suitable fluid. Slikolene Pro RSF 2.5 is roughly the same weight( 14 centrastokes) as Maxima 5 wt.( 14.8) but the viscosity index of Silkolene is close to 400 and Maxima is like 150. I will take the Silkolene any day.
Street riders like myself will see the some (possibly the greatest) contamination of the fork oil, due to rain getting swept into the oil. Obviously, the more back and forth sweeping the bushings do on the fork legs, the more surface imperfections are going to be polished out.
I would expect that the greater the motion of the fork (how much of the available sweep) the more opportunity for gunk to end up in the oil.
Does the resulting slurry have the same centrastokes and viscosity index as fresh, pure oil of the highest possible quality? I'd have to say, no. Is there enough of a difference that I should be able to feel it, when replacing sludge for fresh? My gut says the difference would be slight, in my use. There, but not tremendous.
I've ridden street bikes with completely shot forks..... as long as the springs are appropriate, they will allow commuting with moderate risk..... just having the seals bad, and leaking oil on the legs, tires, brakes is enough to park the bike before you have to part it out.
Bad fork oil, like bad coolant is an insidious fault. You don't know it's bad, until it bites you.....
I may be talking myself into getting the forks serviced.