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 from manufacturers 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.
When mixing hydraulic fluids ensure both grades are of the same formulation or brand
BMW updated BMW fork fluid & submitted an SIB to the NHTSA in 2013, it applies to all BMW motorcycles
The pdf can be downloaded from NHTSA, it is NHTSA document ID 10147846 rekevant extracts are below
The telescopic fork oil with part number 83 19 2 182 459 and a certified viscosity of SAE 7.5 has been superseded by the new telescopic fork oil with part number 83 19 2 408 581 and a certified viscosity of SAE 11.5. Despite the difference in viscosity classification, the two grades of telescopic fork oil have similar properties but are of different chemical composition, so they cannot be mixed together.
Change of viscosity class:
SAE is a standard for engine oils and transmission oils. Today's high-quality fork oils are hydraulic oils and are graded by the standardized unit of measure CentiStokes (cSt) at 40 °C. The SAE class is only conditionally meaningful as regards the damping characteristics of the forks.
Viscosity of the two products compared:
Part number 83 19 2 182 459: SAE 7.5 at 40 °C has a centiStoke grading of 33.7 cSt.
Part number 83 19 2 408 581: SAE 11.5 at 40 °C has a centiStoke grading of 32 cSt
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.