GavinJuice said:
Just talked to another well advised amsoil dealer about running the medium #10 fork fluid, he said large heavy v-twin bikes such as harleys run 10W. He seemed quite surprised that this bike ran a 20W.
Maybe the Amsoil dealer is
"well advised" about the lubrication requirements of relatively modern Harley (and other) fork types, but what needs to be taken into consideration is that the Roadholder fork dates back to 1947, when
any kind of hydraulically damped telescopic fork would have been considered to be a relatively new idea, the damping oil viscosity required being rather more to do with the damper design and not really to do with the weight of the machine as I see it, and
*motorcycle fork oil* would not have existed at that time, the only real choice for a commonly available fork oil would have been engine oil, the lightest type generally was SAE20 grade, so I expect that is why the Roadholder forks were designed to use SAE20, or
light motor oil.
Oil specifically formulated for forks certainly didn't become commonly available (in the UK and probably many other countries) until around the late seventies, (ATF by that time being specified for some motorcycle forks) which, in any case would really have been too late for the Norton Company.