Drain Plug Shenanigans

[MIPS]

When I got my Eagle it originally had a copper washer on the oil drain plug and was hard to keep it from leaking. I replaced it with a rubber/metal washer which doesn't require as much effort to tighten it on and it hasn't leaked since.

Well about two years ago I did the oil and forgot about the washer and reefed it on pretty hard. It didn't leak but the last three times I took the plug out and then back in all I can say is that the threads felt "weird". You couldn't insert or remove it by hand because it would bind in the threads and was difficult to keep it from cross-threading. Visually the threads in the pan looked okay and so did the threads on the plug but the inner shoulder had worn down. I had to replace the washer because I had more or less ruined it by that one over-tightening. Last night it went for the annual change again and this time while it came out relatively easily it would not snug up when put back in. Thing is the threads on both sides still look okay so if I've pulled the threads out of the pan it's not immediately evident.
What's confusing me more is I added an extra washer to the plug and while it didn't completely snug up like it should it did thread-in firmly and it hasn't leaked overnight, so...?

If the damage is done, sure I'll admit it but I'm confused why it's still grabbing enough threads to seal if I just add the washer. Worn drain plug? Everything needs the thread chased? No idea if this has been fitted with an oversized plug.

[89 MJ]

I'd start with chasing the threads and changing the drain plug, but not until the next oil change.

[Taylor]

Had that happen with my wife's car 13-14 years ago. She got it from her mom. Her mom always had general maintenance done at a certain local shop. Not a good one and it was part of a national chain.
I went to re install the drain plug, added a rubber washer. Realized quickly that the threads were toast. Being that this was the first time I touched the car I knew how it had happened.
Ordered a heli coil kit, drilled it, threaded and installed the reinforced coil and a new bolt with a rubber washer. Never had a problem afterwards.
I think i ordered the kit from granger. Might be able to find one elsewhere. It came with everything needed to do the work except the drill.