My best experience was to take a detergen system cleaner DSC, an oil soluble additive. added 2 to 5% in the hydraulic, you work normally for 24 hours and then change the oil. Good DSC remove all kind of deposits, varnishes, and contaminants, and disperse them into the oil to be drained when changing oils. Please let me know if you don't find any since I could give you some recommendations.
If the filters get clog fast, you may opt for holding the cleaner in the oil just for few hours changing filters frequently and then change the oil.