If it hasn't been mentioned already, with some of the 5 bolt pumps you have to glue in a small extra piece of gasket material or the oil.pump will not make pressure. I had to do it on my engine. I went from zero to 60 on oil pressure once I added that little piece of gasket. I think it's in one of docs oil pumps. https://www.flickr.com/photos/13718356@ ... 499249739/

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“Buy the ticket, take the ride.”
― Hunter S. Thompson

Not necesarily. You can find a discussion about some filters that have an internal tube, to keep the oil inside the filter at shut off... The early slants had one tube in the filter connection. I don't recall in what thread that info is, but you might have a filter with no oil (when the engine stops...). Hopefully the filter was wet.

UPDATE: Found the discussion thread about the tube:

viewtopic.php?f=19&t=63493&hilit=oil+filter+tube

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/6 Dodge Charger 69
...there's nothing like using the "Search" link above to solve most of your problems...

Well, it turned out to be the oil pump. It failed. Luckily it didn't damage the cam. I had to have the engine pulled to replace it. Oof. New oil pump installed.

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What kind of filter? How long since last oil change?
Also remember that on these engines, the oil filter hangs "upside down" from how it goes on just about any other engine. Did it all drain out while removing?