Hmmm... you say it came on pretty quick, but had been fine up to that point. Doesn't sound like a bearing clearance issue to me. Sounds like an oil pump issue, and more specifically, a relief spring issue.
I don't know about an engine overhaul being required (yet), but it's probably safe to say that you're gonna need to pull the pump if no amount of Marvel Mystery Oil or the like is able to 'free it up' again.
Good luck, and here's hoping you don't have to pull the engine!
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Eric Mitten
'74 Bonneville 4dr Sedan (455/TH400/2.93 open)
'72 LeMans GT (455/M-13/3.23 [8.5"] posi)
'71 GTO Hardtop (400/TH400/3.07 12 bolt posi)
‘71 GTO Convertible (455HO/TH400/3.23 posi)
'67 GTO Coupe (455/ST-10/2.93 posi)
Deuteronomy 8:3
