Before 1936, Harley-Davidson motors operated using a “total loss” oiling system. This meant that the oil was never recirculated back to the oil tank and instead it just ended up on the ground after going through the motor. It does pass through the primary to keep the primary chain lubed on the way out, but every ounce of oil you pour into the tank eventually ends up as a trail of drops and drips that leave a greasy trail back to your motorcycle.
Even with oil constantly belching out of the motor, the factory service manual recommends that you change the oil about every 750 miles. In my experience you never reach 750 miles between oil changes, especially if you are using modern oil control rings on your pistons. While using oil control rings sounds like a good improvement, it allows oil to build up in the crankcase which you then must drain more frequently since too much oil creates extra drag on the flywheels. Knowing that changing the oil would be a constant maintenance headache, Harley’s engineers did have the foresight to design the crankcase with a built-in drain to make this a quick and easy job. Located on the left rear side of the engine, a spring-loaded oil valve allows you to drain the oil with just the push of a button. Read more.