Low oil pressure 99 Buick LeSabre

I overheated my last motor in my 1999 Buick
LeSabre custom with a 3.8 V6 and we replaced it
with a 2000 3 Pontiac Bonneville engine which is
also a 3800 series 2 V6 but it still has low oil
pressure once it starts heating up we will replace
the oil pump the oil switch sending unit I just want
to figure out what the problem is because I was
told and oil plunger could have not been replaced
but I do not know what to look for please help me
figure out what's wrong with my oil pressure I'll
take any suggestions