A liter and a half sounds lilke a lot of oil but then five weeks is a long time. At this point, I wouldn't worry too much about it. However, after you work through some of the other issues you might check the clearances in your oil pump.
It's a satisfying experience to drink a Guinness Stout or three while you patiently lap the oil pump body to precision tolerances, especially if its cold and rainy outside.
I have owned many bikes with the dreaded wet-sump-problem and have never drained the crankcase prior to starting. However, after starting, I do let the bike idle for several minutes, alowing the pump sufficient time to scavange oil from the crank case and circulate it back to tank.
There is an excellent procedure for checking/lapping the oil pump in various Norton work shop manuals. If you don't have this information in any any of your manuals or documents, let us know and one of us will get it to you.
Just a reminder. Lapping the body of the pump to reduce excessive side clearances is normal maintenance of this style pump. But be advised it adresses only one of 4 pump leak-thru paths and should not be thought of a complete pump maintenance, but rather as a improving a dying pump.
4 leak paths.......
These three feed out to the crank ......
gear ends to body (lap body)
pump gear teeth end to body
pump gear to gear clearance
cross cavity leakage down the shaft to the scavange side of the pump and back down to the sump
If the tank is completely empty, drain the sump and refill the tank before starting. Failure to do this will result in lack of oil flow to the bearings until the scavenge side of the pump can pick up the oil and return it to the tank where it is available for the pressure side.
If the tank is low but oil is visible, you can go ahead and start the engine and let the scavenge side (which is much larger) refill the tank.
I pretty much just drain em, unless I have recently ridden em.
When I return the oil the to the tank, it gives me a good reference as to the oil level, I know the sump is empty and I can see if I need to add any.
It also allows for easier starting, no smoking, no oil discharge ...
It's not that big a deal.