Can't tell you what the problem is, only that I think that something may be wrong because my 1958 B33 hold the oil (40 monograde) in the tank for many weeks with very little escaping to the sump. Same with my 1959 Matchless G3 (same oil), unlike my 850 commando which can hold the (50 monograde) oil for several weeks or drop it all to the sump within 2 weeks, just seems to depend on its mood!
Beware of taps and valves.