Yes, this is an “oil thread”, but it is not the same’ole Amsoil thread so don’t glaze over just yet or open the Peanut Gallery. fork oil - also know as suspension fluid I decided to do the maintenance on my forks and I was in the process of selecting fork oil. My shock will be farmed out so I focus on the front end below, but the info should apply to shock oils. As I dug around for information the more I read the more I began to wonder. After quite a bit of digging I didn’t find what I was looking for on ADV or KTMTalk, so I spent some time on the Web trying to figure them out. It seems there are some misunderstandings about fork oils. They were likely started by the industry themselves, using official standards that do not apply well to fork oils (SAE lubrication oil weights like 2.5wt, 5wt, 10wt). Not only that, but many fork oil manufacturers don’t bother providing the correct standards that best characterize what are primarily hydraulic oils (SUS/VI and ISO-VG). Then stir in some internet gossip, footloose and fancy-free chat, and straight-up shootin’ from the hip bullshit… and you can end up with a few folks at best throwing up their hands in disgust or at worst parading around with empty PBR bottles like they found The Holy Grail. Now I am not claiming I know everything about fork oils; I was just trying to understand them so that I could choose decent fork oil. It seemed to me this stuff wasn’t well known on ADV or KTMTalk so I thought it might help someone out if I posted it. Is this information critical? Well I think riders have been enjoying their bikes without it, so in that sense no it’s not. But it could help you optimize the operation of your suspension, and at the very least give you reliable performance indicators for the different brands of oil. If you have something to add or correct please post it. (gratuitous fork pic) Looking for info So at first I dug around on ADV and KTMTalk. From my limited survey of fork oil threads related to my bike I came up with these: Can someone explain fork oil? Confused on what to run, especially for sand.... – has a discussion of fork oil weights LC4 fork oil level? – an ATFer chimes in, and some interesting and reasonable rebuttals Fork Maintainence, complete or routine? – more ATF WP fork expert needed – more ATF 640Adv'03 Fork Oil Change? – hmmm, check KTMTalk atf instead of fork oil, will it hurt? – more ATF, with a link to: Mobil 1 ATF Fork Fluid, Other brands with less friction?? – yet more ATF Well I didn’t find a good comparison of fork oils like I have found on brake fluids (such as THIS). I did not even find a big discussion of fork oil, but I did find conflicting opinions and a lot of ATF. This made me curious so I started looking around for more information. More searching… So I went fishing on the Net. Without having the correct search terms it took awhile but I finally came across this: The Secret World of Fork Oil… I think it is a good introduction to fork oils, but more importantly it gave me some good search terms. Particularly the term “viscosity” and the tests used to measure it. The big take-home point I have to offer is this: even though there is a “stamp of approval” (i.e. SAE) doesn’t mean the viscosity scale is appropriate for fork oil. Is it completely wrong? No, but my reading led me to believe SAE weights gloss over important details on fork oils. The article then went on to introduce me to a scale used for hydraulic fluids, which is more appropriate for fork oils. It also said why the SAE scale is used on fork oils: you already know it and are comfortable with it. Digging deeper With the terms from that article I was able to find a bunch of information, good or otherwise, on fork oils and hydraulic fluids in general. Here is some of it: Let me just start with what I think is one of best links, which includes a summary chart of the various oil brand’s viscosities and grades! Peter Verdone Designs (but remember nothing is perfect… ) Here is a very good article on the effects of oil “weight” and height by Jon Stoodley (with great info on suspensions in general) And a bit of info on viscosity from: Maxima Oils, a discussion of fork oil “weights” in a chat forum, and of course the ever present ATF proponents in another another chat forum. (more wanton fork art) are we there yet? What did I learn from all that digging? I can’t say for sure , but at least I know the SAE rating on fork oils is not the greatest scale and can be misused. Verdone said (in his link above) there are different SAE tests and something marked 5wt can actually be more viscous than another oil marked 7.5wt, etc. Yes, given the right circumstances you can be your own grandpa… so don’t laugh this off. No, stop laughing... anyways, I now believe hydraulic fluid measurements and related scales/grades are more reliable indicators of how specific oils will behave in your forks. So what did I look for in fork oil? Start with the manufacturer’s recommendation, or the recommendation of the suspension shop that reworked your forks. They will likely list an SAE oil weight - fine - the oil manufacturers that do not use that scale will note the SAE weight equivalent of their oils. The recommendation is important, because it will give you an idea of what viscosity will work with your fork’s valve setup. With that in mind one of the links above said to look for fork oil that has a Viscosity Index of at least 150. On that scale the higher the number the less an oil's viscosity will change with temperature. This is good because it means the your forks will not change how they react to terrain as they heat up or cool down. For example, HERE is the spec sheet of the fork oil I bought. There might be more to look for but I will have to leave it to folks more knowledgeable than me. For example, Verdone (linked above) states that the ISO-VG grade is a more reliable measure of oil viscosity, but as you already know recommendations do not include an ISO-VG. There might be other details such as shear stability and such but at this point my eyes started to glaze over and I decided maybe it is good enough to rely on the expertise of a good synthetic oil manufacturer. I hope that helps someone out there.