Oil maintenance is divided into two subjects, oil filtering and oil changing. Many crane models have a return filter located in the top of the hydraulic reservoir that filters the oil before it is returned to tank. Other tanks are less accessible (mounted below a truck bed) and have a remote stand-a-lone filter located in the return line.
Return filters fall into two categories; cleanable and replaceable element. Tank filters are often cleanable screens that removed, cleaned and returned to their holder. Replaceable elements can be either a disposable element installed in a housing or a housing and filter media in one piece like an engine oil filter.
Oil changing is recommended at least once a year. It is better to change the hydraulic oil twice a year in spring and fall to change it after and before the extremes of our seasonal climates. It is effective to change oil before winter to remove traces of moisture before water droplets can form ice droplets. It is also effective to remove any winter moisture condensation from the oil before you experience the extreme summer temperatures.
Recent high technology enhancements in filtration have added the option for a renewal of existing hydraulic oil. Your oil is cycled through a special series of filter steps that removes many more impurities than on-board filtration can. Your oil is then returned to your crane with an additive boost to replace the additive chemistry broken down in use.