There's no way to really get all of the fluid out of most machines that run on hydraulics. Just soo many places it goes to. Changing the fluid in the reservoir and then running the system through a kidney loop while cycling all functions is about the best way to try and get everything "fresh". It may be a bit overkill though. Cat has these kidney systems and IIRC after various types of hydraulic repairs they automatically run the machine on it.
The fluid itself might be doing okay as long as you don't have a lot of contamination. Send a sample off to get it checked if you want and they can give you an idea as to what its like. At that age though, if its never been done it may be getting close. Actual hour reccommendations are all over the place. Some of our farm tractors say 1,000hrs others say 3,000hrs. Our Cat SSL's want you to use SOS to go to 6,000hrs+ otherwise change at 3k IIRC. As with anything it really depends on the situation and an analysis will give you the best answer.
The gauge drops suddenly or slowly drops over time? As the oil heats up the pressure can drop. Does everything still function normal or do you start noticing you are loosing functions and such? Testing the pressure is just one side of the equation, a hydraulic tester that will read flow and pressure will give you a good idea of whats going on at least on the auxillary circuit.
