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At 300,xxx miles and poorly maintained as you say, have you performed a compression or leak down test?
Cold or normally hot engine, full battery charge, and throttle wide open with fuel injection depowered (also pull the HT ignition coil wire). Or remove the infamous 25-A ATC fuese in the engine bay (hopefully in a sealed fuse holder, or relocated as:
http://cleanflametrap.com/emfuse.html
You have verified the PCV breathes freely yet with engine oil fill cap on PCV is sealed.
Test cylinders 1 by 1 or all four at one time using a thread in compression tester. Crank until reading raises no higher.
https://www.brickboard.com/FAQ/700-900/EngineMechanical.htm#CompressionTestProcedures
If a low cylinder, as in the FAQ, some oil dribbled into the spark plug hole temporarily raises the cylinder compression, suspect excess compression loss.
I'll disconnect the PCV pipe that connects to the intake accordion-like hose at the throttle body. Pinch the end of the small PCV vacuum line between the PCV flame arrestor housing where it secures at the intake manifold. Pinch the intake manifold end shut. Blow the the large PCV hose. With oil cap off, you exhale and easily displace air through the oil fill hole. Restore the oil fill cap, and should be very difficult to blow through to displace air. No air should come out under the breather box. Strong lungs and maybe some slight air sound inside may escaping through either manifold.
You have the unsecured, yet resting atop its mount on the engine running test. PCV vacuum draw through the sump should secure hold the unsecured oil fill cap in place (or glove does not inflate). Check oil fill cap gasket condition and that the tabs that engage the tapered.
With freely breathing PCV if the oil fill cap rattles or nitrile or other rubber glove expands, indicates compression pass by the piston and into the oil sump. If neglect, and using mineral oil, and / or driven in short drives that does not heat the engine up (though coolant TEMP guage shows normal), suspect compression rings that are stuck and do not float as excess carbon stuff prevents ring from floating as they should so that the compression rings disallow engine excess combustion getting passed the rings and into the oil sump. I've read some (Captain) Marvel Mystery Oil through the spark plug hole, and let set for a bit (I dunno how long). Hopefully the rings are eventually free and float properly again so they seal as best as can be expected.
150-170 PSI for each cylinder on 1988, I guess?
Hope that helps.
Sleepy MacDuff.
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