Fork Oil Type ? : LUSENET : Mountain Bike Hash Forum : One Thread

Hello, I have a RockShox Judy C , fork , and I discovered after i bought it , that it's system of Hydracoil (open bath hydraulic) , losses oil. Can I refill it with auto oil , or what are the similar oil products . Tks, Radu

-- Radu P. (, June 18, 2003


Hi Radu,

You'll need suspension fork oil. Motorcycle fork oil works fine, and is cheaper. Fork oil comes in different weights; Judy C oil is 5 weight. Use a heavier weight oil if you need more damping.

For detailed instructions on how to service your fork, check out this link:


-- Joe (, June 18, 2003.

I have checked , a gas station , and they have only 2 types of oil . one is hydraulic oil with no gradation , and another is engine oil with gradations like 5W40 ,20W40, 20W40 , the first number is the viscosity the second the temperature. So the first number is the same as the "5 weight" from the oil fork type ? is this oil good ? tks.again

-- Radu P. (, June 19, 2003.


I'm not sure if you can use hydraulic fluid or engine oil. One reason not to use it is if the oil causes the seals or other rubber parts in the fork to swell. Perhaps someone could explain the the differences between brake and transmission fluids, lubricants and suspension fluid?

If I were you I'd err on the side of caution, and get stuff intended for suspension forks. Decent bicycle shops should stock fork oil in various viscosities, or alternatively try a motorcyle repair shop. I bought a large bottle of 20wt fork oil from the motorcycle dealership near the Section 16 roundabout in PJ for about half the price I would have paid for a much smaller bottle of Rock Shox fluid.


-- Joe (, June 19, 2003.

About fork oil, it is very similar to hydraulic oil. The requirements for seal protection are the same. Alos very important for both is the resistance to foaming (or foam formation, please excuse my english). Be carefull, the classification from SAE like 5wt, 10wt, 15wt is not the same as most of hydraulic oil ; viscosity 5W seems to be about 20 following ISO 3104, perhaps 30. That could be correct for most of forks or suspensions systems.

I hope to have helped. Fred

-- Fred Regnier (, September 15, 2003.

Addition to the previous ; 20wt seems to be close to 60 to 70 following ISO 3104 (hydraulic oils).

The most common you can find is the one used in fork lifts to carry palets. See industrial supplies.

-- Fred (, September 15, 2003.

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