Use power steering oil or any ATF, both have suitable qualities to use in a hydraulic jack, and most of our sheds/workshops will be harbouring a spare or leftover quantity of either type. Put the oil in through the "bung" on the body of the jack, make sure the jack is fully down/ compressed and laying horizontal, fill the body/reservoir to the top, then replace the "bung", stand the jack up into its working position, vertical, then close the release screw and pump the jack up, open the release screw and push the jack piston down, this will release air from beneath the piston, tighten release screw and pump again, repeat this operation a few times before placing the jack under working load.
It is definitely a clever marketing ploy to put some basic hydraulic oil into a one litre bottle and sell it for a big price as "hydraulic jack" oil, but that's business!