One thing that should be pointed out here is that the industry standards that apply to other oils DO NOT apply to fork oil. One manufacturers fork oil of lets say 15 wt are not the same viscosity as another manufacturers 15 wt. ie. Bel Ray 15 wt is not the same viscosity as Screamin' eagle 15 wt. Fork Oil - 16oz. Type E 5wt. Type E is for "Showa" forks.Part # 99884-80 Fork Oil - 16oz. Type B 10w. Fork Oil - See owners manual for recommended style. Part # 99880-73 Screaminâ€™ EagleÂ® Performance Fork Oil - A heavier oil (15wt) provides extra damping in performance application. Part # 99881-87 . Works well to take the dive out of the front end on breaking. Screaminâ€™ EagleÂ® Racing Fork Oil - Designed and blended for the severe conditions on the race track (20wt). Not intended for street use. Part # 99909-93R There are also other options for tuning the forks on your bike. Bel Ray makes a host of different weight oils to do this also available in 5wt, 7wt, 10wt, 15wt, 20wt and 30wt. www.belray.com/consumer/productpages/hpfo.html Stiffness of forks is purely personal preference. A heavier fork oil will not create a stiffer fork, but it will slow down the damping response. This is good for handling, up to a point. A fork with heavier oil won't dive as quickly when you grab a handful of front brake heading into a turn. It also won't wallow as much in a fast turn. All this is good, up to a point. A fork that's too heavy provides a harsh ride and can cause a loss of traction over stutter bumps in a turn. You need to decide what's best for you. If you're happy with the way the bike handles now, then stick with factory recommended weight of fork oil which in most Harleys is a 5 wt oil.