I put an engine from an '83 ZX in to my '77 Z, and it runs great, until now. Would it be the fuel pressure regulator? The regulators are different from '77 to '83, so I changed the fuel harness to my original (which matches the ECU) and am curious if this should work. Or is there any other possible way gas could be entering the oil? Any knowledge is appreciated.

my guess would be piston rings.