Why not? Simply because most gaskets and seals are not subject to system pressure. The valve cover gaskets, front and rear crankshaft seals, and oil pan, are subject to CRANKCASE pressure. The oil filter seal, pressure sending switch, and filter housing, are subject to SYSTEM pressure. I think what mallen wants to know is if a leak can develop under hard driving conditions but not when driving normally. If all systems are functioning normally, such as crankcase ventilation, then no oil should leak from anywhere. I'll bet the leak is always there, it just gets worse when driven hard. Keep us posted