I know it’s been said before, but I’ll repeat it for those that missed it...
If you fit the Comnoz / cNw breather at the back of the case, or even easier, if you fit the Comnoz sump plug replacement reed valve breather, these will rapidly return access oil in the sump back to the tank. By rapidly, I mean within a few seconds of starting up.
It’s not a cure per se, but it removes the symptoms of the problem.
The only time you need to intervene is when it’s been stood so long that the entire tank has emptied into the sump (ie, if it’s stood over winter).
For me, its a much more comfortable, and simple, way of living with a Norton than putting anything in the oil feed line.