Sweet! Your Clark should have a dipstick on top of the differential behind the mast. Needs to be checked running as I recall. Even though the dipstick doesn't state it. My favorite is tractor hydraulic oil as transmission oil.
There is also an inching "slave cylinder" on the top of the transmission as I recall. It has a brake line going to it and a bleeder that requires a 1/4" wrench. I have seen these hang up due to rusted/ broken internal springs and cause a transmission to slip. Might want to take a look at this.