These old engines differ from modern designs in that they have a "dry sump", in other words unlike engines where the oil is in the sump, it constantly circulates into the engine and back to the oil tank and there should be only a minimum of oil in the crankcase.
But on standing for a while, there is a tendency for oil to drain down into the sump from the tank, so called wet sumping, and you are blessed indeed to have a motor which does not do this. Used daily the system can cope. Left for weeks, the tank will be empty, the oil now hiding in the sump. You top it up, next thing oil everwhere.
Think of the system as two distinct parts...a conventional pressure side supplying the plain bearings, but also a "scavenge side" returning oil back from the sump to the oil tank. The clever bit is that the scavenge side has greater pumping capacity, so the sump in effect should always be more or less empty.
The problems arise when the balance between these elements is upset, and on the Forum there is enough to read for weeks on how this has affected the health and well being of our esteemed members.
To add to Musky's advice, while the sump plate is off you will see a small ball valve below the engine flywheel, which fits through the gauze in the sump plate. This valve is the start of the return side "Scavenge" system.
Check that the ball is not stuck, is free to move up and down and treat it to a good dose of carb cleaner. A stuck ball means the return side is effectively blocked, and the first thing to check with no return to the tank. Common on engines left standing for a good while.
The gauze obviously needs to be nice and clean, and with a new purchase I hope you don't find any assorted bits of metallic debris...the first sign of trouble. Failure to scavenge is another cause of "Baffled, confused and covered in oil", another topic to search on the Forum.
With the engine running and good oil return to the tank, that's a sign that all is well, and a check that most of us make on every start up, a sort of good luck ritual. See Cols' post above, the flow does vary depending on the balance of the flow and return systems.
You may also notice none of your modern spanners fit, neither do any modern metric fasteners. BSA use good old Whitworth, BSF and Cycle threads (CEI)
Welcome along, your troubles are only just starting....
Swarfy.
