the problem is the continuous rise and fall of the air pressure in the crankcase, cam chest and rocker boxes. That the pressure changes are caused by large volume of air moving in and out of the crankcase as the pistons travel up and down. As a result of this air flow and the changes in pressure in the cam chest, air also flows in and out of the rocker boxes through the cylinder head oil drain passages. At sustained high speeds the constant in and out flow of air in the rocker boxes interrupts the free flow of oil from the cylinder heads to the cam chest . In fact a small quantity of oil, pushed up and down by the pressure changes, is trapped in the drain passage, effectively blocking the passage so that no oil can drain out of the rocker boxes .
When enough oil has accumulated in the rocker boxes, it will start to blow out the breather passage to the air cleaner and ends up all over your bike, your passengers new boots, and in extreme cases on your rear tire .
Reason for Oil Blow By: More air is displaced by the ascending and descending pistons when a 95" and larger kits have been installed, simply because there's bigger pistons moving up and down. Sustained high rpm riding aggravates blow by because more oil is being pumped into the engine since the pump is turning faster. The rocker boxes become overwhelmed with too much oil that it cannot separate the oil from the escaping air.
Some stock displacement engines have oil blow-by problems even when ridden moderately, but that is not the norm. Blow-by does not happen immediately because the oil buildup that causes it takes time to occur. That's
why blow-by sometimes doesn't occur until after sustained high-speed riding.