A lot more info needs to be provide to conclude or asertain the possible causes.How is the oil pressure on cold startup,how long before the oil pressure drops,what is engine temperture,oil temperture,and what is the cause of the heat? As mentioned is the filled block the issue?What do the bearings look like?A lot of guess work can be avoided by check the above mentioned issues.Oil passage cracks,plugged or restrictions as well as cooling issues can cause similar issues.We have seen one bad main cap,a crack in the block main bore at the oil passage as well the issues at the lifter bores cause the loss of oil pressure.An easy analysis of the fluid tempertures(which get hot first)would be a good start.Check and recheck all oil pickups,pumps,filters.lines and passages.Check and recheck all cooling systems and passages also.We
find many issues are caused by simple over looked things.