Judging from rider's reports on this site, you should stick to 15W if you are below the 185# range (with the correct springs). The damping could be a bit harsh with 20W at 165#.
Consider Traxxion emulators and damper rods. They are worth the money. Stock springs should be OK for street use if you're talking 160# in gear. If you're 160# "neat" then you probably could use a tad stiffer spring.