Sounds like it might be running out of fule under a load JMO are you running a electric pump or mechanical pump ? and just what do you measne by no oil pressure under a load 0 or exaclty how much are you calling none ? could be the picup tube or just not full of oil and when it is under a load it moves away from the pickup tube. JMOhi out there i have a 1971 460 mark111 in my 1981 F150 lots of oil presure when cold idle hot no presure when you step on the gas engine dies let of gas engine picks up no lifter noise