I would not recomend straying from the factory recomended fork oil weight, especially not if the forks are allready on the stiff side. The factory oil is 5wt, only use 5wt fork oil or the Kayaba "01" oil (which is about 5wt). Look up the oil level spec in the manual, it will give a range, lower oil levels will make your fork feel softer as it will not build up as much air pressure when it is compressed due to the greater air volume. Be sure to set the sag on the shock and play with the clickers to help with the stiffness.
good luck