One of the reasons I suggested finding the 3 manuals had to do with hydraulic filters and screens. During the production run of the 100 series, there were several changes and updates. In the manuals, there will be listings of serial number breaks where those changes are made. Since you don't have a serial number plate, you would be limited to identifying YOUR tractors filter/screen combination by a visual check. (Filter is accessed through that "round plate" where the dipstick and flow control are located. Takes a contortionist to REACH it, but it's there)
I've cleaned out the hydraulic sump on a few older Fergies and MF's. I always flush them out with diesel fuel before re-filling if they appear overly dirty. Drain everything that will drain. Lower the 3-point. (With position control AND draft control levers in down position. That shuts down the hyd. pump) Pour in a few gallons (about 3 usually does it. (More is better, but with the RIDICULOUS price of diesel nowdays....) Start the tractor, then drive it SLOWLY around for just a few minutes. Diesel is a lubricant. It won't hurt anything, so long as you don't work the tractor, or run it long. (No plowing the back 40 with diesel in the sump!) Then drain the diesel from the hyd. sump. Let it drip dry for a few hours. You will be amased at the volume of crud that washes out. THEN change filters and re-fill with clean, new oil.