Why does my 2000 Yanmar VIO70 lose hydraulic power when it gets hot?

[USVeteran]

I have a 2000 Yanmar VIO70 mini excavator that's giving me some hydraulic trouble. After about an hour of operation, the hydraulics slow down almost to nothing and the system gets very hot. I'm currently using 35-weight clear hydraulic oil.
Could it be a weak hydraulic pump, a relief valve issue, a clogged filter, or the wrong oil causing the problem