Oil return - 1956 International 490cc

... till there's been some fed down from the tank. How many times have you turned it over?

Be certain the return line is primed by feeding oil into it.  And get some oil in the sump so that it can return. I fit a clear piece of oil line into the cambox feed so I could be confident all was well up top.

You could start by removing the cam cover. Squirt lots of oil inside just to make sure it's safe.  And turn the engine and watch for oil delivery upstairs before sorting out return.

Do you have the oil pressure tell-tale?  (Not all do).  Is it popping up?

And remember...the oil feed and delivery pipes arrive at the opposite levels on the crankcase from other Norton Singles.