Is it ok to lubricate exercise equipment hydraulic cylinders?
Replace it. There is nothing to "lubricate" (it's already full of oil) and if there was it would not increase resistance.  You have some sort of failure in the valving/orifices that restrict the flow of the oil inside the cylinder and convert motion to heat.

They are more directly analogous to shock absorbers (as on your car) than "hydraulic cylinders" and the vast majority of shock absorbers are not rebuildable, including the ones used on most exercise equipment.