I'm not a mechanic but I suspect that the mixing trans fluid is really just an issue when used in transmissions. In transmissions there are tons of different paths for the fluid to go through and the pressure by the fluid is what engages different gears (plates) - different consistencies would probably have very wonky results. I believe hydraulic jacks are a much, much simpler design where the pressure simply extracts / retracts one cylinder (for each leg but since each is controlled semi independently, there's no real consequence).