Responding to your post, I have had several D series with live PTO and live hydraulics.
There are two main enemies of the D series PTO clutch and one main enemy of the hydraulic system---
1. Slippage caused by the PTO clutch adjusted too loose
2. Improper PTO start-up procedure
3. For the hydraulics, it is dirt in the unfiltered hydraulic oil.
As we know, the PTO clutch needs to handle momentary peak loads without slippage--- such as when using a bush hog in brush. Also, most live PTO clutches on many tractors are not designed to handle repeated high torque start-up loads at high engine RPMs.
The PTO clutch needs to be adjusted so there is a strong “detent feel” when the lever engages. The D series engine has good low torque so it will engage most PTO loads at low RPM. The live PTO clutch should last a long time.
The time era of the D series tended to not use hydraulic oil filters. Dirt tends to enter hydraulic oil mainly by—
1. Engaging dirty quick couplers
2. Worn out hydraulic cylinder rod wipers.
As we know, the late D series has a separate, small hydraulic oil reservoir and no fine filter. I don’t remember but the suction pipe may have had a screen to keep out big chunks
The D series hydraulic system was good for its era of time. Its gear pump is more tolerant of dirt than either vane pumps or piston pumps. The gear pump is positioned down low so it keeps its prime. A gear pump is quite tolerant of cavitation. The roller chain drive is extremely tolerant of dirty oil. However, spool valves are not very tolerant of dirty oil.