If this happens there are three variables that can cause it.

(1) External leak. By cleaning and tensioning your unit you will see, in time, oil leaking from some point.

(2) Internal leak. Usually after some time the seals can degrade and allow oil to leave the pressure system and return to the reservoir.

(3) Leaky valves.

In all three cases something is wrong. Whether it's been damaged or just wearing out, it is time for service.