This baffles me. The last time I changed oil on my 1979 W150, what seemed to be the original (or very old at least) drain plug was clearly stripped and would not tighten back down. I got an aftermarket replacement from parts store. It tightened down as much as I could want it to. I don't put a torque wrench on it, but I put plenty of torque on it. Next time I was under truck, I noticed oil seeping from plug, so I got out wrench, and was able to tighten it again. Now, each time I drive the truck, it clearly loosens up some again and I'm able to re-snug it. Again, I can tighten it plenty so it does not feel stripped.

Do I have bad plug or is pan bad? As far as I can tell, it's the original oil pan (360). I've never had this problem. Either something is stripped and won't tighten, or it tightens good and stays.

Thank you.