Good find with the bronze filter.
You can use 10W-30, 10W-40 or regular pink hydraulic fluid. I run 10W-40 in my 642.
I added a couple of gauges to my 642 so I know what it is doing....I added an engine oil pressure gauge and a hydraulic charge pressure gauge. Charge pressure is easy to get out, you can install a tee in the line where the idiot light hooks up at the charge block on the top of the pump assembly. Depending upon what setup you have, I believe the stuffing pressure of the hydrostatic pumps should be between 100-115psi. This is the discharge pressure from the hydraulic pump (Vickers 10 series pump, the pump at the very front of the machine) as is stuffs or charges the hydrostatic pumps. The hydrostatic pumps (usually Vickers TA1919 series pumps are the two pumps that control the left and right side drive. As was stated earlier, the hydraulic vane pump is easily damaged if you let it run dry. A gear pump is much more robust, but all 642's shipped with vane pumps.