If you have a Micron gauge to monitor with, once a pump starts losing performance, you will probably be surprised at the improvement by simply changing the pump oil.

Many years ago I worked in the instrument shop at North Texas State University Biology/Chemistry department. Along with keeping the Spectrophotometers and Liquid Scintillation Counters going, Vacuum Pumps were part of our responsibility. It was very common for students to turn the wrong valve on their apparatus and suck some serious contamination into the pumps. We had a Micron gauge that we tested with, and simply changing the oil would provide drastic improvements.

These were big, two stage SERIOUS vacuum pumps that cost a ton. Sure wish I had one.