Quite possibly your crankcase ventilation plumbing is all crudded up. When you load it, the blow-by increases and with a plugged up vent system, it backs up from the crankcase into the intake 'directly' instead of through the oil/vapor separators in the rocker covers(normal route of exit)

I suggest adding a pint of Marvel Mystery Oil to the crankcase and run it to help clean out the ring grooves.