Nice score.
First things first, grind off where it says "property of Safety-Kleen" stamped in the back surface where the pipe that holds the lid open is attached
In order to ensure longest life, fill the tank to where the pump is covered with plenty of solvent to ensure it won't run dry and that there is plenty of solvent to cool the pump.
The pumps should pretty much run forever, I've had mine for more than 30 years and the original pump is still running strong.
The Safety-Kleen guy who called on my mother's truck shop in the 70s showed up one day saying that he had gotten a great deal on refurbished Safety-Kleen machines that were being sold outright and did her shop want one. We bought several of them and one of them made it to my home shop. As a 17 year-old hot rodder, this was heaven.
Of course a month later another guy from Safety-Kleen showed up, told us the last guy had disappeared and that he was the new rep...
I still have that parts washer!