Oils can and do loose their viscous properties over time. However, I’ve not heard of a “recommendation to change oil” if there has not been a problem related to the oil itself. Over the years the only time I’ve seen oil changed is when the hydraulic oil looses its lubricious properties and begins to bind the hydraulic piston to the ram seal (through a glazing process) resulting in a chattering in slow speeds or an outright stalled cab seized in the packing gland for the same reasons, OR water contamination of any sort or degree. The former of the two reasons results in callbacks in which depending on your contract cost the company money.
There might be other answers to your questions, but that’s my two cents on the question based on my 34 years in the elevator trade, but I’m no expert either.
NOTE: Replacing the oil in your 5 stop elevator has a few more twists than simply replacing the oil. Purging the 70’ jack and hydraulic lines of old oil. Draining the reservoir tank, 6 or 7 empty barrels and hazardous disposal etc all add up. But $11K? I don’t know. But you can as the owner get other estimates for the job IF you choose to move forward.