We are all used to specifying our fork fluid using the SAE specifications of 5/10/15/20 weight. The SAE specification though is not suited to suspension fluids. It is designed for motor oils not suspension fluids.
The SAE rating system allows for a wide range of viscosity and changes of viscosity with temperature within grades.
ATF has long been used for damper rod forks as it is manufactured to a tight viscosity range suitable
for fork usage.
A problem with ATF however is the additives in it may result in painted parts which it comes into contact with may not be able to ever be repainted, especially modern plastics. Leaking fork seals can spray fluid over body work.
The ISO VG (viscosity grade) rating system is a far better specification to use. The VG ratings specify viscosity at 40 and 100 degrees centigrade and provide a "viscosity index" to give a guide for viscosity changes with temperature. The higher the index the more stable the viscosity with temperature variations.
The ISO VG ratings for most suspension fluids are available from the manufacturers web sites.
A pdf of information collected on line is available for download. Fork Fluid Information
There is a wide variation in viscosity of fluids from each manufacturer marked by the SAE ratings. A glance at the specifications above will demonstrate the differences. The second aspect worth noting is the superior temperature stability of fluids rated for shock usage. A viscosity index of 200-400 is preferable.
It is possible to blend intermediate grades of fluids but the resultant grade is not a linear grade. It depends on the specific gravity and viscosity of the fluids being mixed and there are variations in SG between fluids of the same nominal grade. The formula used to calculate the viscosity of the blend is called the Refutas Equation.
The fluid calculator page provides calculators for specific gravity and the viscosity of blended fluids.
There is a wide variation in the viscosity of fluids used in each of these fork type. Damper rod forks in general utilise both heavier springs and oil viscosity compared to the newer more highly tuned cartridge forks.
Many cartridge type forks use a viscosity of 16 cSt@40C. (Except for Ohlins R&T)
Many damper rod type forks are specified for a viscosity of 34 cSt@40C.