If it's leaking down with a load on it, there's a problem with the piston seal(s). If it's failing to reach its full travel, then adding oil will help. Any lightweight oil like power steering fluid or EP-32 hydraulic fluid should work OK for refilling. Long-stroke replacement jacks aren't expensive or hard to find- - - -try Northern Tool & Supply or maybe Harbor Freight.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!