I'm not sure what your preventive maintenance schedule is like, but some often like to plan their activities in conjunction with the seasons, and spring appears to be the quarter of choice when it comes to cleaning. That's right, I said spring. It's only a little more than a month away, so get excited!
If I were to plan annual maintenance on hydraulic systems, the following are areas on which I would focus.
Oil analysis. Although I recommend more frequent oil sample analysis, doing it every spring would be a good start.
Schematic and BOM updating. It seems to be inevitable that machines are hybridized by upgrades and swapouts over time. I recommend that you update your hydraulic schematic and bill of materials for your machine once a year. This will prevent annoying head-scratching during failures because nobody can figure out why the schematic doesn't match machine layout and function.
Spares inventory. If you are a larger manufacturing outfit, your stores room will have one or more racks for hydraulic components. I recommend that you inventory those items, and also confirm the requirement of those items. You may have obsolete valves or pumps on the shelf with no current use in your facility. They could have been from old machines or existed prior to upgrades that made them obsolete, but are just taking up space in your shelf, confusing maintenance persons.
Training. Spring is as good time as any to do some hydraulic training to freshen your team. As mentioned in the main article, hydraulic knowledge can leech out the brain like reverse-osmosis, so a yearly day-long seminar will keep everyone confident in their ability to deal with hydraulics.