Hey, that's great!
It was a quick test I used all the time while browsing the salvage yard. You'd be amazed at the number of "good" jacks that end up there. I mean Really good jacks. If you do clean it out and the solvent comes out pretty dirty, you might want to repeat that part a couple of times. It's hard to drain it all out of reservoir but do the best you can. Any small amount left in the reservoir will mix with the jack oil just fine. What ever you do, do NOT use lacquer thinner.
Also, ATF will work just as well for jack oil. Same stuff, just a different color. (Better additives too.)