1988 244 DL N/A with about 300k miles with distributor mounted on the engine block right above the harmonic balance. It's a Chrysler ignition.

Main problem for the past month: poor gas mileage consistently and intermittent when moving from stop or driving up to about 20 mph backfires then won't move for about 10-15 seconds, then takes off. No problems with acceleration above about 20 mph. Exhaust smells horrible; I can't describe well, maybe gas, but it's definitely not rotten egg smell. There's no smoke that I can see.

My engine oil is black after only about 4k miles since last oil change. Spark plugs are fouled

About a month before this problem started, I did the following work:
timing belt/water pump and all oil seals (leaking)
cleaned fuel injectors off car with nice spray pattern
changed o-rings
changed fuel filter
new fuel pressure regulator
new distributor cap, rotor, spark plugs, and spark plug wires.

In other words, I did not have this problem until a month after doing these things, so I'm thinking that and the fact that the car runs fine above 20 mph means that the timing belt is on correctly (but I could be wrong).

Someone suggested that my piston rings might also be leaking oil.

Here's the other problem: inside the distributor cap is oil, but not outside it. I checked FAQs about the distributor, am unclear, and wanted to talk doing this out for my first time so that I don't botch the repair:

https://www.brickboard.com/FAQ/700-900/ElectricalIgnition.htm#DistributorandShaftSealReplacement

Since the oil is inside the distributor, I would need to replace the smaller o-ring, but should probably replace all of them?

I removed the 13 mm retaining bolt in the top rear. The distributor is seized on. How do I remove it and not change the timing or reinstall the distributor in the wrong tooth position?

The distributors for my car fit only 1985-1988 240s, according to Autozone.

I can't find the o-rings on Amazon or Ebay; where can I get replacement o-rings for the distributor?

Thanks