The filter should be changed when the pressure drop through it reaches 10 – 15 PSI. Lacking an indicator, the rule of thumb is every 6 months to a year, depending on use.
The oil should be changed if it starts to turn milky (due to water contamination), starts to break down (overheating, varnish deposits), or smells like it has burned. The oil in a hydraulic system is usually good for a long time, perhaps years, because it usually isn’t exposed to burning heat like oil in an engine. It’s much more important to change the filter. Sending an oil sample to your oil supplier is the best way to tell if your oil is contaminated or worn out.
