When the shim stacks are open the fork oil viscosity has very little influence because there is a lot more flow area than when it is just cycling through the bleeds at low fork shaft velocity. Moreover the low speed rebound control is already marginal through the bleed so going to a lighter oil is going to exacerbate that poor area of control.
These are cartridge forks we are dealing with, not damper rod forks.
So the cheap fix ( lighter oil ) is not a fix at all. The piston ports are simply not big enough to flow enough oil volume at abrupt bump velocities ( fork velocity, not bike velocity)
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