No way your gonna get oil to only one side.
Couple things come to mind.
1) I know some filter don’t have an anti drain back valve. Could have lost prime. Corrective action- loosen filter and have someone start engine to see if you have oil flowing right away or if it takes a minute.
2) oil pan gasket. Pan gasket has o-rings built in that feeds filter inlet and outlet. Verify oil pan bolts are tight and didn’t back out. If they are loose and still no pressure after tightening- could have blown out o-ring seal on pan gasket.
3) camshaft thrust plate. I’ve only seen this once but guy forgot to tighten down cam thrust plate, says he did but must have backed out [emoji849], no oil pressure to lifters.
4) blown out plug on block but that one would be pretty obvious
5) bad pump. Shit happens. Can’t rule it out. I would think your idiot light would have come on and you would have seen it. Low or no pressure on passage coming off pump would be seen on gauge or idiot light. If you have pressure on gauge and no oil coming out the pushrods, that points to thrust plate. I’m sure he will find it. Might just be something stupid. Did he see any traces of rockers getting oil or is it like “holy shit man, this thing hasn’t been getting oil up here this whole time”?
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