Again...fork oil is not what stiffens the fork. That is the job of the spring. The shock absorber dampens the rebound of the spring. That rebound is controlled by the oil and the viscosity of that oil controls the dampening rate. Heavier oil is used to dampen heavier (springier) springs. That's why Honda recommends ATF with a viscosity of around 7w for stock springs and Progressive suggests you use 15w with their springs.