In stock configuration, most all used oil ends up in the flywheel cavity.
Pressured oil in the rocker boxes splashes oil to the exhaust valves and enters the crankcase through the head drains.

The rotary breather valve functions to relieve crankcase pressure and scavenge oil from the crankcase.
It also controls the flow of oil in the lubrication system.
The breather valve does not pull any oil or air out of the crankcase. Piston downstroke does that.
The breather is nothing more than a window of opportunity for air and oil to escape the crankcase.
It's timed to engine rotation, opens just after piston downstroke and closes at Bottom Dead Center (BDC) 71< or 85°-90° (ABDC) after piston upstroke begins 72-76 respectively.
Piston downstroke provides “pushing power” to usher the oil into the rear oil slot in the case and to the oil trap in the gearcase at the breather window.

The FSM states that it is the flywheels that sends air and oil into the scavenge path to the gearcase.
This isn't the whole story. The oil slot (or galley) is taller than the scraper. Oil has to be “ushered” there by downstroke and wheel motion.
The volume under the pistons on downstroke pushes oil thru the slot to the oil trap at the breather gear.
The wheels alone are not powerful enough to push oil from under the scraper and into the slot pushed against the breather valve.

The flywheel scraper provides forward windage in the crankcase but can't scavenge oil by itself.
Most of wheel windage is above the scraper.
Oil gets behind and under the scraper from gravity as well as the wheels.
Anything that is thick enough on the wheels to be slung off will hit skirts, piston unders, walls, crank bearings etc. and fall below the scraper.
Anything that slides down the wall will fall below the scraper.
Gravity holds it there under the scraper. The wheels are done with it until the oil shows up on the rear end.
The force from downstroke pushes oil to the slot. However, on the that end, higher windage and vacuum CAN pull it back up instead.
The scraper won't stop that and air can only leave thru through the oil trap.
So the scraper keeps oil from perpetually wrapping the wheels.
Piston downstroke pushes both oil from under the scraper to the oil slot and to the oil trap and exhaust air to the oil trap as well.

The breather window is rotated to where the window is exposed to the crankcase area and oil and exhaust air pressure is pushed into the center of the gear.
From there, oil and air is spun out the top of the hollow gear into the gearcase and further oil separates thru stages to lube moving parts on from there.
Opening and closing times of the different breather valves are below but the exact opening-closing points may vary between engines.

As the pistons rise, the breather window is closed and vacuum begins building in the crankcase until the pistons reach TDC.
Then the whole cycle starts over again.

The pics below show the flow from the flywheel cavity to the return pump (applies to 76< only).

• The slot that runs East and West on the rear wall leads to the oil trap near the breather gear.
  (oil galley, oil slot, oil passage, maybe other names for it)
  That is the way out for both air pressure and pressurized oil from piston downstroke.
  
• The oil trap is a compartment (or temporary trap) that is sealed off from the gearcase by the breather window being closed.
  Both the two different breather windows open after the pistons start decending.
  The pistons create positive air pressure before the breather opens (later down the cylinder).
  So when the breather opens, oil and air are shot out of the gear faster since the piston has now built up some juice behind.
  So air and oil is “trapped” there momentarily by piston pressure during downstroke.

XL / XLCH scavenge oil path out the of the crankcase.

Oil slot. 1)	Half of an oil slot. 2)	Oil trap. 3)
Another view of the oil trap. 4)	Breather / oil pump installed. 5)	Oil enters the pump here. 6)

1957 to 1976 Sportster production pump breathers:

Breather Gears 16)	Some of amf's signature quality control. These are 3 - 72 breather gears.17)	A closer view18)

Left is the unmodified 1/4 speed R valve. Right is the 1000 stock.	R model is the left one.

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