I've heard those rumours as well. If the valve sticks it could result in serious engine damage. By the time you've noticed the low oil pressure on your guage I expect the damage would be done. One way to work around this is to fit a tap to the oil feed linked to the ignition so that teh bike cannot be started when it's closed.
Or, here's the view of the Norvil company in the UK:
Wet Sumping Solved
Your Norton Does Not Need To Wet Sump
Many people are turned away from owning and riding classic bikes, especially Nortons, because of the problem of oil leaks and wet sumping. Well, I can tell you here, no word of a lie, my 650SS does neither. Wet sumping is an issue many riders have found with their classics. There are four simple steps that have been followed to eliminate that potential problem on my 650SS. When all are used together, your problem should be solved forever, meaning more time can be spent riding your bike rather than emptying the sump and refilling the oil tank.
1} Firstly, you need to use the correct Monograde oil (SAE 50 in summer and SAE 40 in winter).
2} Secondly, you need to leave your pistons on compression, to do this, slowly press down on your kickstart about an inch or so, until it reaches the top of the effective swing. This has the effect of raising your pistons and the big end journals to the top of the barrel and so oil has further to go before it can drain out.
3} Thirdly, some Norton's can have an anti-drain valve fitted inside the timing cover, (AFTER the oil pump), this prevents the seeping of oil down onto the bottom of the crankcases. On Dominators & Commandos Pre 131257, a modification can be made to your timing cover, if you send it to us, (see workshop services code WB12). On Commandos with engine number 131257 onward, a new timing cover, (066161A), can be fitted to solve the problem - see picture below. ** PLEASE NOTE ** This is NOT the dangerous type of anti-drain valve, which is fitted by some people into the oil feed line above the crankcases. Anti-drain valves fitted into oil feeds, always cause oil starvation to the big ends for a brief time when starting the bike. If you use a tap instead and forget to turn it on, it will totally blow up your engine).
4} If the above three steps have failed to solve your problem, then you probably have a faulty oil pump. Oil can flow backwards through the oil pump if it is in need of attention, most oil pumps can be repaired and restored quite simply – mine was. The fourth step, is to have o-rings fitted to your oil pump shafts to stop oil draining from the feed to the scavenge side.
Good luck sorting it out.
Whippet