I always add ZDDP for the first couple of oil changes then just quality oil and filter. ZDDP additive wouldn't have caused any failure like you had. I like NAPA Gold or WIX filters, I wouldn't put a Fram filter on any of my engines. I hadn't thought about a dropped valve, but it could be, so you'll want to pull a valve cover. I still think, based on the bent rod cap, that a rod bolt failed or came loose. If you're lucky an additional turn of the rod journals, new bearings, and a new rod should put you back on the road.
The 7-8 rod is the one that fails most in an AMC engine, since it receives oil last and gets the least. Did you add the lifter valley oil mod? If you did, check that you didn't screw the fitting in too deep and block the oil passage. If you completely disassemble the engine, which I would recommend, look into the cam bearings that Bulltear sells that reduce oil to the cam journals. They still get enough oil, but it increases oil to the last main/rod.
Bob
tufcj
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