I recommend changing the oil in any hydraulic system at a minimum once a year. Doing so; this will help the life of any system in more ways then one. For example, it is getting close to winter and if you have any water in your oil, the water will freeze and can either slow your system down or not allow it to operate at all. Another reason to change your oil is if you have water in your system, water will rust your metal sealing surfaces and reduce the tolerance needed for seals and will wear your seals out so that they leak or not allow your system to have its full potential usage that is required. Oil is a lubricant, if your oil has contamination in it, it will destroy your system. Down time is costly, but poor maintenance of any hydraulic system is much more costly, and doing a simply oil change can save you money in the long run.