I've fix a number of hyd jacks and engine hoist cylinders over the years if they are not leaking fluid simply by draining as much oil out of them as I can and filling them with fresh 15 wt motorcycle fork oil. I measure the oil I take out in oz and use that for a start. Sometimes it is close enough sometimes I end up adding an 1 - 2 oz more.

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2) replace with a new, Chinese-made aluminum floor jack with a life expectancy of 5-20 years and cost of $100-250

I abuse the heck out of a number of Harbor Freight products with fairly good results. I have a few of their aluminum jacks. The key to prevent destroying one of them is to stay away from side loading the jack. This will bend them.

Over the years in shops we had better quality floor jacks fail and usually when I did an autopsy I found pitting on the shaft/piston.

We had good luck rebuilding those that had clean, non-pitted surfaces. Once that rust broke the surface even if you removed the rust you had a depression. Had to remind the guys to occasionally lubricate them and always lower/retract them after use.

Note that most seals in the cheap rebuild kits aren't of OEM quality.