You may already know this, but worth mentioning.
Fork oil will change the action of the forks, but valving is a better solution. However, both of those only affect the speed that the fork piston travels through the stroke, both up and down. That will never overcome too heavy or too soft of a spring for your weight.
Springs keep the bike in the correct part of the valving. The valving is in stages and too soft of a spring will put the forks in a later stage of valving and often make it feel harsh and bottom easier, mistakenly making you think it needs heavier oil or stiffer valving....usually that is in the springs. Too heavy of a spring makes the rebound feel harsh and chattering on the initial stroke of compression.
Getting the right spring rate first makes oil and/or valving MUCH simpler.
I think you can find fork springs for the WP X50s somewhere. Like said earlier, they came on 640s and many heavier riders with gear used them so someone made heavy duty spring rates....find some still may be hard. Not impossible.
it is easier to ask for forgiveness than permission