If it didn't have proper oil pressure it could have been the oil pickup tube, I had someone swap my engine for me and the 2 bolts that attach the flange to the pump stop and get pretty tight, it seems all good till you drive it (I drove mine with a 1mm gap at the flange for 100 miles, luckily I didn't ruin the engine, just caused accelerated wear in the head), I could not figure it out so finally 2 weeks later I was going over every bolt with a torque wrench and those 2 bolts got another 3 full turns out of them
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Remember simple mistakes on someone elses part can be costly, its better off to do it yourself right the first time