sounds to me like its been run around the farm the last 20 yrs on the same old oil and sludge, and who knows what the farmer used to add oil when it needed it. pull the valve cover off and you'll probably find the oil drain back holes plugged or partially plugged with crud. when you get it running at higher rpm's down the highway the oil cant drain down fast enough and comes out the down tube. some of the oil drains into the lifter gallery and those holes could be plugged too, fills the lifter area up, fills the head up and theres the drop in oil pressure, nothing left in the pan. i've cleaned a couple of these that were so plugged its funny any oil returned to the pan. 1 more idea, if the oil cap isnt vented and the engine has decent blowby it will actually create so much of a draft in the downtube that it sucks the oil thats splashing around the rockers right out the down tube.
