Lets talk fork oils....

Hey everybody,

The main reason I'm posting this is that I recently found one of my fork seals to be slowly leaking. As everyone knows, problem = upgrade opportunity. I found replacement seals easy enough (https://www.ktechsuspensionusa.com/p-20107-k-tech-suspension-fork-oil-seals-wp-pair.aspx), but fork oil selection (even finding OEM) was a little more involved. I've already done 90% of the research, so hopefully this can help someone else out, but I'm also looking for opinions and verification of my findings from people who may be more knowledgeable.

Right now, I have 2 options: 1) Minimize downtime, by only replacing the leaking seal and using OEM fork oil; or 2) Use this opportunity to upgrade fork oil and remove/disassemble both forks. So I want to answer 2 questions:

1) Can anyone verify what fork oil comes in the stock bike? (Mine is 2018 US version)​

My repair manual says SAE 4 oil (PN is obviously Motorex, like all stock fluids seem to be). Seems the '4' is weight, even though there's no 'W' after it. 4W is rare, but found some here (https://www.summitracing.com/parts/mxu-154038). However, the electronic OWNERS manual, lists SAE 5 and SAE 4, without any indication of which is in the bike. My hard copy that came with the bike only lists SAE 4, so that's likely what I'm going with if I do option 1. Just be aware, that this seems to vary

Repair Manual:
Owners Manual:

2) Does anyone have an aftermarket fork oil recommendation?​

I've seen this asked a couple times here, without real informative answers. Not looking to replace springs or buy an Ohlins cartridge, just swap fluid (if it's noticeably better than the OEM). Right now, it seems like there's a little stiction, although not problematic, and I wouldn't mind it being a hair stiffer. Any recommendations? Since I know someone will ask: I'm about 175 lbs. without gear, commute daily, but also do several track days per year. Please provide reasoning behind your selection too.

For non-experts asking the same question: Here's where I found some good reading: http://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid
-Weight number means nothing - It varies by manufacturer (ie: 7W at one supplier could be lighter than 5W at another) so ignore that.
-Viscosity is what matters - Dig that up and use that instead of weight. If you want stiffer, obviously go with a higher viscosity.
-Different forks are made for different weights/viscosities - So don't vary too much from OEM. Your friend may like 20W in his forks that were designed for 15W, but 20W in your RC will probably feel like solid rods.
-VI is also a good metric - This is how much the viscosity changes with temperature (ie: how consistent it will be throughout your ride). Higher VI = more consistent. Low VI = will soften more as you ride and it heats up. However, this is more important for rear shocks. Front forks apparently don't cover a wide temperature range, so this is less critical.

If I don't get any recommendations, I'm probably going to go with OEM to minimize work. However, down the road, I may experiment with a slightly higher viscosity (bump from Motorex 4W to 5W), but keep it close, as I'm not sure what's "too much" or "too little," and the bike is rideable as is.

Please share your knowledge, if you have any.


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I have recently done some searching on fork oils (for my old Duke 200) and made a shortlist of 5W oils (W does not stand for weight).
Maybe helpful to you.
The VI=viscosity index number is a very important detail as this signifies how well the oil behaves with temperatures.
Another variant is the amount of oil you put in a fork leg, less than recommended will give you a "softer" feel while a little more stiffens it up (little by little though)

As for fork seals I think the best ones are the SKF green seals.


  • Fork Oil 5W.pdf
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Thanks guys, this helps.
(W does not stand for weight)
I know, but everyone still seems to refer to refer to it as weight. I guess I should probably stop.

Thanks for the list. There's a very comprehensive one in the link I provided above, but their numbers seem to vary slightly from yours. It appears they're just tested to different spec's (ASTM D445 vs DIN51562), so qualitatively they should still be the same. I haven't dug into whether one is more accurate than the other.

Another variant is the amount of oil you put in a fork leg, less than recommended will give you a "softer" feel while a little more stiffens it up (little by little though)
This is good to know. In that case, I may just go with OEM and try this method to add stiffness. Then if it's a pain getting into the first fork, I can abort operation stiffness, just put in the recommended amount and get back to riding.
As for fork seals I think the best ones are the SKF green seals
Unfortunately, I already ordered the K-techs. :( Seems they're OEM for most Jap bikes though, so hopefully will work fine.


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After replacing the fork seals, it is relatively easy to add or distract oil, starting with 100ml increments, you may fine-tune later.
Just loosen the top triple clamp bolt, unscrew the top fork cap and use a syringe with a transparent tube to add or distract oil.
This way you can precisely measure the amount added or taken out as well.
Also check, by putting a ty-wrap around the lower/chrome part of one fork leg, the amount of travel you use as this will also tell you if you're too stiff or if it bottoms out.
Yep, I dug through the repair manual and a fluid swap seems to be pretty simple, so I'm going to plan for swapping the fluid in both.

After some more research and digging through options, I've picked out Motul Factory Line 5W as the most probable candidate. I couldn't find any good comparison videos or sound research on various fork oils (just a few sites saying "this is the top 5 or 10," without good reasoning). My reason behind the selection:

1. Reputable, big name brand that is readily available.
2. Fully synthetic - I'm not sure how critical this is. Everyone seems to suggest that synthetic is the way to go, but most brands don't even seem to offer a synthetic, which makes me think it may not be that critical for fork oils....but it won't hurt.
3. Should get me 10-15% additional stiffness in the temperature range that matters - I wanted to avoid making a big jump and going too far. Also, seems that fork oil doesn't heat up much (only rear shock), so the 40C is really the number you care about, not 100C. In reality, using the two points to extrapolate back to ~7C (45F), and looking at the 7C (45F) to 40C (104F) viscosity range is probably the way to go. That should cover you from start of a cold ride to a hot track day. In this case, I'd see ~10-16% stiffness increase across the board. The OEM oil and Motul are tested to different specs, which provide slightly different numbers, so there's a few percent of uncertainty there.

The leak I have right now is very slow, so I'll do a little more riding with it and verify how much has been lost when I do the swap. I'll throw a zip tie on now for a baseline, then start with the recommended amount of fluid (15.55 oz.) when I switch to Motul. I'll report back on how it works out. At this point, might be after the new year though.


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Follow your own advice and look up the CentiStokes value of the oil - the actual weight number of oils means little as has been noted. The cSt value should be on the packaging somewhere - use this to compare brands.

I run a lot heavier oil in my track bike but have also put in much heavier springs, I've probably over done it as mine is too stiff.