And if I remove the screw will all the oil drain out?
No. The oil line that the Factory drilled was from the small chamber behind the flywheel oil scraper that collects the oil that is flung off the flywheel that runs back to the scavenge end of the oil pump. Thus, the oil that is in the sump is drained out of the engine from the drain plug on the primary side. All of the oil in the sump has to be carried by the flywheel over a cast-in wall that has a scraper that does not touch the flywheel but guides most of the oil on the wheels to the chamber.
All the oil in this somewhat hard to get to scavenge chamber flows up to the rockers then back to the oil tank. It is only important for you to make sure that all the oil in this small chamber needs to get back to the tank to continue circulating. Without the grub screw, no oil returns to the tank.
The second revelation is that the flywheel has to lift all of the oil into the scavenging chamber from the sump. When the engine is running, this is done automatically and effortlessly by the engine. However, when starting the engine, if the flywheel is swimming in a sump full of oil, the added drag of that oil slows down the flywheels enough to prevent the engine from starting. This is called "sumping" and is universally despised by owners. Draining the oil from the sump will cure this impediment to starting and it will leave any oil in the scavenge chamber in place, so the oil should start returning to the tank fairly quickly. If the scavenging chamber is empty, it will take a long time to fill, particularly if the sump has been emptied of oil.
David