Truck Repair: oil lose, rocker shaft, main bearing cap

Question
why is the front of the rocker shaft oiling more then rear

Answer
What engine?? I see you have the M11.The frt camshaft bushing oils the frt rocker shaft & rear camshaft bushing oils the rear rocker shaft.IF the plugs have fallen out of the rocker arms that will loose oil pressure.But depending on what year M11 you have I would still check the #6 main bearing & cap at next oil change & check the oil pressure regulator while the pan is off.