Remember that the 40 in something like 10W40 is the measure of viscosity at 210 deg F working temperature and the 10 is the measure at "cold". My guess is that the temperature of a multi-grade oil used in forks will never get to anything like 210. Instead it work more like a 10+. It seems to me to be a bad idea to have a variable damping oil viscosity. It also seems a bad idea to use oil that does not have the anti-foaming agents that ATF and fork oils use to limit performance degradation in repeated sustained use. That said, my preference is to use a "10" fork oil in my 1993 R100R and "7.5" in my 1994 R100RT and I change oils at least once a year to keep the forks and oil as clean as reasonable.
Current - '74 TR5T : '93 R100R : '99 Gran Canyon : '06 ST3s
Past - Variety of British, French, Italian, Czechoslovakian, and German bikes from the '70s, '80s and '90s
R100RT Mark screwed with this post 11-08-2012 at 02:53 PM