This is a very general guideline for fork oil weights. One thing you need to remember is that, like all fluids, fork oil viscosity is reduced as the oil temperature increases... so, under racing conditions, the oil will heat up and thin out more. On the street, it will stay relatively constant. This is one reason race shocks (and superbike forks) have reservoirs to help cool the oil. They also provide a space for nitrogen to pressurize the fluid. This helps keep foaming and cavitation (air bubbles) through the valve to a minimum.
100-150lb rider with .85-.90 spring = 5 wt
150-180lb rider with .95 springs = 7.5wt
180-215lb rider with 1.0 spring = 7.5-10 wt
215-240lb rider with 1.05 spring = 10-12.5 wt
240lb and above with 1.10 spring = 12.5-15 wt
There will be some overlap in these numbers based on usage but that's about how they stack up.... BTW.. to make the half weights (I.E. 7.5 wt = 1/2 volume of 5 wt mixed with 1/2 volume of 10 wt)
Now for the disclaimer! Different fork manufacturers use different "port sizes" in their valve assemblies (requiring different viscosity) so this is not "chiseled in stone" Please verify all your oil wt questions via a manufacturer's representative!