a lift that slowly bleeds off when the pump is off is not a huge deal.. if it will hold up a minute or more under load, that's fine for 95% of farm work.
it's possible the cyl bore is not perfect and that's the leak.. or could be thru a check valve or relief, or control valve.
heaveir oil may mask the problem a little bit.. however.. you also risk cavitation in the pump, and a blown pump, unless it was designed to pump heavy oil. for instance.. a ford N with a scotch yoke pump was specifically designed to pump 90w oil. pumps on the newer machines were designed around utf fluids.
sc gets cloder than fl.. gear oil in a hyds system might as well be a lump of grease when it gets cold.. that is not healthy for a hydro pump....
your call.
I don't have a manual for the dexta.. so am just putting some general info out.
most other ford products are specifying a m2c134 spec utf style oil in hyds...
