Hello everyone.
"I want to change my fork oil, but which weight should I choose?" "Is it true that #10 from one brand can be thicker than #10 from another?" When you service your own bike, choosing the fork oil viscosity is surprisingly confusing.
Fork oil weights such as "#10" or "#15" are only each maker's own guide.With different makers, oils with the same weight can have a different actual thickness (kinematic viscosity).
In this post, I put together a table of the kinematic viscosity at 40°C of fork oils from the main makers, explain how to choose, and show the oil I used in my Honda XR250 (the Japan-market XR250 BAJA / MD30).
Conclusion: compare the kinematic viscosity at 40°C, not the weight number
- Even oils labelled "#10" range from 32.7 to 48.1 mm²/s at 40°C depending on the maker (see the tables below)
- When you switch to another maker's oil, choose one with a similar kinematic viscosity at 40°C
- If in doubt, start with the same thickness as the genuine oil specified in your service manual
What is fork oil viscosity? The basics
How viscosity affects the ride (thicker vs thinner)
Fork oil viscosity affects how fast the front fork compresses and how fast it rebounds.
Higher viscosity (thicker)
The fork moves more slowly, and nose dive under braking is reduced. The bike feels more stable at high speed and in sporty riding, but you feel bumps in the road more harshly.
Lower viscosity (thinner)
The fork moves smoothly and soaks up bumps well. The ride is more comfortable in town and on tour, but the front may not hold up as well under hard braking.
The difference between the weight (#10 etc.) and the kinematic viscosity at 40°C
A fork oil weight such as "#10" is only each maker's own guide. To compare how thick oils are, look at the kinematic viscosity at 40°C (mm²/s).
For example, even among oils with "10" in their name, the kinematic viscosity at 40°C varies this much:
| Oil | Kinematic viscosity at 40°C (mm²/s) |
|---|---|
| Yamaha Suspension Oil G10 | 33.2 |
| Kawasaki Fork Oil G10 | 32.7 |
| KYB G10S | 41.2 |
| WAKO'S FK-10 | 33.6 |
| WP Fork Oil #10 | 48.1 |
Because the figures differ like this, the trick when switching to another maker's oil is to check the kinematic viscosity, not just the weight.
Fork oil viscosity chart by maker (kinematic viscosity at 40°C)
WAKO'S (a Japanese chemical maker) publishes a fork oil viscosity comparison table in its general catalogue, so I copied out the kinematic viscosity at 40°C for each maker. All values are in mm²/s. Product names are the Japanese-market names translated into English.
Honda
| Oil | Viscosity at 40°C |
|---|---|
| Cushion Oil Special II 5W | 16.1 |
| Cushion Oil 5W | 16.2 |
| Cushion Oil Special III 10W | 36.1 |
| Cushion Oil 10W | 34.7 |
| Cushion Oil Special IV 5W | 13.8 |
Yamaha
| Oil | Viscosity at 40°C |
|---|---|
| Suspension Oil M1 | 18.5 |
| Suspension Oil S1 | 15.8 |
| Suspension Oil G5 | 17.5 |
| Suspension Oil G10 | 33.2 |
| Suspension Oil G15 | 47.4 |
Suzuki
| Oil | Viscosity at 40°C |
|---|---|
| Fork Oil L01 | 15.3 |
| Fork Oil 10G | 33.3 |
| Fork Oil 15G | 46.8 |
| SS-05 | 15.5 |
| SS-7 | 16.3 |
| SS-8 | 35.2 |
| SS-19 | 15.1 |
| SS-47 | 36.2 |
Kawasaki
| Oil | Viscosity at 40°C |
|---|---|
| Fork Oil KHL15-10 | 14.9 |
| Fork Oil KHL15-11 | 16.1 |
| Fork Oil G5 | 17.1 |
| Fork Oil G10 | 32.7 |
| Fork Oil G15 | 47.2 |
| Fork Oil 2F | 22.7 |
| SS-7 | 16.3 |
| SS-8 | 35.2 |
| SS-19 | 15.1 |
| SS-47 | 36.2 |
KYB
| Oil | Viscosity at 40°C |
|---|---|
| Fork Oil KHL15-10 | 14.9 |
| Fork Oil KHL15-11 | 16.1 |
| G10S | 41.2 |
Showa
| Oil | Viscosity at 40°C |
|---|---|
| SS-05 | 15.5 |
| SS-7 | 16.3 |
| SS-8 | 35.2 |
| SS-15 | 19.6 |
| SS-19 | 15.1 |
| SS-47 | 36.2 |
| SR-6 #5 | 15.7 |
| SR-6 #10 | 37.8 |
Öhlins
| Oil | Viscosity at 40°C |
|---|---|
| Front Fork Fluid No.2.5 | 14.9 |
| Front Fork Fluid No.5 | 21.8 |
| Front Fork Fluid No.10 | 38.4 |
| high performance R&T43 | 18.2 |
WP
| Oil | Viscosity at 40°C |
|---|---|
| FRONT FORK 5 | 23.5 |
| Fork Oil #7.5 | 33.4 |
| Fork Oil #10 | 48.1 |
Harley-Davidson
| Oil | Viscosity at 40°C |
|---|---|
| Hydraulic Fork Oil Type”B” | 32.1 |
| Hydraulic Fork Oil Type”E” | 39.1 |
WAKO'S
| Oil | Viscosity at 40°C |
|---|---|
| FK-01 (T500) | 15.4 |
| FK-10 (T530) | 33.6 |
| FK-20 (T520) | 53.4 |
Source: WAKO'S general catalogue, "Fork Oil Viscosity Comparison Table" (PDF, in Japanese). The values are those in the catalogue at the time. Check each maker's own documents for the latest figures.
The original table also gives the ratios for blending WAKO'S fork oils (FK-01, FK-10 and FK-20) to match each maker's viscosity.
By the way, when I saw this I thought, "I'll try WAKO'S fork oil! Oh, about 3,000 to 4,000 yen a bottle. Pricey, but let's try it." Then I noticed each bottle is only 500 ml... So that's 6,000 to 8,000 yen per litre... Not something you buy on a whim...
The fork oil I used in my XR BAJA
The XR250 service manual specifies Honda Ultra Cushion Oil No. 5.
I have two front forks for my XR BAJA. I bought a spare set to use while the inner tubes were away being re-plated (in Japanese).
So, after checking viscosity and price, I filled one fork with Yamaha Suspension Oil G5, and the other with a blend of A.S.H. fork oil #11 and #30.
In the tables above, Honda Cushion Oil 5W is 16.2 and Yamaha G5 is 17.5, so G5 is close to the genuine oil.
I rode on both forks, but I couldn't tell any difference. I am now using the fork with the A.S.H. blend.
You can also blend oils to match a viscosity
If you can't get an oil with exactly the right viscosity, you can blend two oils to match it. For the A.S.H. oil in the other fork, I mixed #11 and #30 in a ratio of 7.2 : 2.8 to match Honda's specified viscosity (16.2). I explain how to calculate it in the article below (in Japanese).
How to choose front fork oil without getting it wrong
Choose the viscosity to suit how you ride.
Mostly town riding: "the same as genuine"
Start with the same thickness as the maker's specified genuine oil. The maker tested and chose it, so it is the safest choice.
Sporty riding and twisty roads: "a little thicker"
If the front dives too much under braking, you can go one step thicker (#15 instead of #10), or blend oils and adjust a little at a time.
If the fork feels too light on an older bike
You can raise the viscosity slightly above the specification. But the cause may be an oil leak or worn parts, so I recommend checking the fork first.
How to change fork oil, and related articles
For draining the fork oil, I hope the article below helps (in Japanese).
For filling the fork oil, I hope the article below helps (in Japanese).
If you also replace the fork seals while changing the oil, check which way the seals go and the condition of the inner tubes.
- Which way round does a motorcycle oil seal go? Exceptions to "markings face out", and how to tell by the lip
- XR250 BAJA front fork assembly (part 2): pressing in the fork seals (in Japanese)
- Caring for front fork inner tubes: which way to wipe, and how to spot pitting (in Japanese)
- Front fork re-plating: cost, turnaround and finish (Toyo Koka, a Japanese plating company; in Japanese)
Tools for changing fork oil
As well as the oil, having these tools ready makes the job go smoothly.
- Oil jug: so you don't spill oil and can measure the amount accurately. One with a lid also keeps dust out
- Fork oil level tool (syringe type): fork oil is basically set by level (height), not by volume
- Fork seals and dust seals: replacing them while you change the oil saves redoing the job when they leak
- Parts cleaner: for cleaning inside the fork and degreasing spilt oil
Summary: compare kinematic viscosity, not the weight number
- How thick a given weight (#) is varies by maker. To compare, use the kinematic viscosity at 40°C
- If in doubt, start with the same thickness as genuine
- A little thicker for sporty riding, a little thinner for comfort. You can also blend two oils to match
By changing the fork oil and matching the viscosity, you can bring the front end closer to how you like it. Use the tables to find the right viscosity for your bike.
*Written by the author (18 years of doing my own maintenance, not a qualified mechanic) from research for servicing my own XR BAJA. Always follow your bike's service manual for the specified oil and oil level.