Hydraulic oil in an excavator

[bastion]

I have a 8 ton excavator. After using it for an hour or so the intake hydraulic strainer gets clogged with oily gunk.
I have to stop to strip it down and clean it out.
Does this mean I have to re-new the oil (95Lt's of type 46) or is there a way of filtering it the existing oil?
Its only done about 1500 hours its a 1995 Sumitomo SH60.

 

[8100]

Bit weird that .Has someone tried sabotage it in the past with water or something like.I would change the oil for new to be safe and filters

 

[Mur Huwcun]

Sounds like it needs changing and a flush out. Do you know any agri or plant fitters/mechanics that do servicing jobs on large plant as you could then easily get some drums of used hydraulic oil to use to flush it out before filling with new oil.