• The MoCo specifies HD Type B or E (and respective equivalents) fork oil depending on year model.

• HD doesn't divulge the highly classified weight of their fork oil. Some say type E is equivalent to 10W, some say it's equivalent to 15W oil or more from other manufacturers. Just keep in mind when changing fork oil, you don't want to mix different used oils together with new oil. Drain and clean all previous oil before installing new oil especially if previous oil is not known.
  • HD has, however, confirmed that type B (0-10W) fork oil is lighter than Type E (5-20). ¹⁾ That being said, if you want to stay with HD oils, you can mix B and E for an “X” weight of your choice to tune your forks. All HD fork oils are petroleum based, non-synthetic oils.

• If you want to use aftermarket fork oil besides the HD brand, it's best to start with an oil weight that has been proven to work in most general situations. The general preferred weight of fork oil is 15(W) on the XLFORUM in whatever flavor you like. So, start by using 15(W) oil as noted and test it yourself. Depending on your weight, the bike's weight including add-ons and baggage, shocks and etc., you might swap to a lower weight fork oil for softer or a heavier weight fork oil for stiffer fork action.
• Generally, I recommend guys start with stock oil height. Test ride, if bottoming out, then add oil. Generally, long forks can add up to 2 oz, short fork 1 oz extra oil. Anything more than that tends to cause hydro locking, so it is best to add just enough oil to control bottoming, which may be less then 1 or 2 oz. Generally, if adding more oil than above to control bottoming, and assuming preload and oil viscosity is correct, the main spring is too light and needs replacing. ²⁾
• I never recommend using less than stock oil levels in the forks. The less oil, the quicker it will get contaminated and/or break down, and less resistance to heat build up, and more likely to have foaming and cavitation problems. I have no ideas what the minimum oil level is that won't start sucking air in to the damper tube. ³⁾

• A lubricant's viscosity varies with temperature. In some areas, the fork oil is already heated up past 100°F by the sun before you start to ride which lowers viscosity and softens the fork action. Likewise, in the winter months, your forks may never reach up to the low end of the viscosity chart below which keeps the viscosity high thus stiffening fork action.
• Below is a sample list of alternative fork oils and their published viscosities. Some aftermarket brands include the words “Type E” or “Type B” on their label. Some may say that's to make you feel like your getting the same quality or “Spec” as HD fork oil (especially since there are 26 letters in the alphabet and they chose the same two as HD). After reading through their TDS sheets, it's obvious that different fork oil manufacturers have a different idea of “exactly” what viscosity that HD “B” or “E” actually is. Most brands have their own special additives to condition seals, control foaming, oxidation, rust, corrosion and etc. These additives play their own role in viscosity at certain temperature levels.
• Most of the data came from SDS or TDS as noted but that can be misleading also. SDS says one viscosity, TDS says something different, SDS viscosity weights obviously reversed (typo), some TDS sheets look to be a cross or makeshift average between a brand's petro and synth weights, incomplete or missing viscosity SDS data….it's enough to make you realize just how tough the oil business' competition is. It is the intent of this article to fill in all the blanks on the fork oil chart, but what is missing was never written by the company(s) that made the product(s).
• Somewhere in some of the oil SDS data you'll find phrases like “The data presented herein is based upon tests and information, which we believe to be reliable.”, ⁴⁾ “No warranty or representation, express or implied is made as to the accuracy or completeness of the data and information in this data sheet.” ⁵⁾
• So viscosity really is a 'buyer beware' subject and it's a trial and error to fine tune to your specific needs/ wants/ riding style. The figures on the chart below are subject to change by the respective manufacturers and they are meant as a learning tool of what you are actually buying as opposed to what you are being told you are getting. Check the labels and current SDS/ TDS data before relying on the fact that “this works for the other guy”. Rely on what works best for you regardless of what the data sheets say.

PJ1 Fork Oil (petrol)²⁴⁾ (Highly refined mineral oil (C15-C50) is 70-99% weight) Synthetic anti-seal swell additive added. Says they are all of High Viscosity Index and Light Red in color. No viscosity weights are published however even in SDS. As a matter of fact PJ1 has the same SDS data sheet (for and that includes) 6 different weights although they publish that they have 8 different weights. Meets or exceeds OEM is published