I have the Parker seal numbers somewhere. I got the seals for a buck or two apeiece, but in qty of 10 that I could use at the time. IIRC, it is lip seal one end and oring on the other end. One on the spol, one in the body. You are better off buying the kit unless you want to do 10 valves....
Because of the lip seal design, the spool has to come out one end, and go back in tyhe other end. Be very careful not to fold or slice the lip seal, but it is a 15-30 minute job.
One drop a minute might not improve. Then ends of the spools are tank port connections. If the return hose and filter are undersized, or if the cylinder is large rod diameter so a lot of extra flow on retract, the T core seems a higher than normal pressure and can leak a bit. Lip seals can take high pressure when pressure energized, but are not leak free at low pressures.
I would order a seal kit to have on hand, but run it until it gets worse.
