Any quality fork oil can be used for the choice of the new fork oil. We recommend that you choose a viscosity class higher than originally, because of the mostly firmer tuning of the WIRTH fork springs. instead of original SAE 10 then new SAE 15. The higher viscosity is slightly thicker and increases the damping.
After removing the original fork springs, drain old oil, rinse briefly with fresh oil, drain. Then fill in the original oil quantity recommended by the vehicle manufacturer in the fork legs. (If only the "air chamber height" is indicated, the stand and dip tubes must be pushed together completely, then fill up with oil until the distance in "mm" between the oil level and the upper edge of the standpipe corresponds to the prescribed air chamber). The fork oil level adjustment set is extremely helpful!
Now replace the original fork springs (dip and standpipe are pulled apart again) and with rod or similar. Measure the height of the oil level to the upper edge of the standpipe.
Now pull out the original fork springs and insert the WIRTH springs with the tight windings up. Now measure the oil level as before and adjust to the same level as with the original fork springs. As a rule, less oil is needed because our springs are usually stronger in the material and have more turns.
RESULT: The air chamber height is now set exactly the same as with the original springs, the air cushion during compression - especially in the last part of the spring travel - by the compression mitfedern as we provided by the manufacturer.