Sorry, I got the compression #'s mixed up with my Jetta diesel that had 500 psi compression on all 4 when last checked. The 83 300D had 420 psi at around 290K per this thread
My compression test results = good on all but #1 which had 400 psi.
Jetta also runs on a 3 tank system and still starts and runs great. It probably is at around 270K with about 60k on veg. This reminds me to do a compression test on the Jetta soon, also pull the injectors (with Monark nozzles) to inspect and pop test them.
Here's a video from 2011
https://www.youtube.com/watch?v=a5AB-Es6MZk of a compression test on one cyl of my 83 300D in under 60 seconds!
The glow plug is already out. Video shows me installing a HF compression tester's glow plug adapter modified with with a quick push on connector, then plugging the 1/8 nylon line into it before cranking. The starter sounds weak. If it cranked faster, the # would probably be higher. At 320 psi, it was probably a cold compression test.