The simplest and most reliable is a manual valve with a switch wired to the horn. That way if the ignition is turned on without the valve in the open position the horn will sound to get your attention.
Mke sure the ignition switch feeds both ignition AND selenoid valve would be better I think. Engine won't start without ignition.
it's normal for a gear pump to leak oil, why because the oil is slippery and the pumps are marginally engineered to withstand normal bolt up torque. the drain down problem is as old as the BSA gear pump. while some manage to finesse drain down to a draw. others seek different solutions.
it appears from most answers here a solenoid is too modern for most traditionalist.
there is a picture on the interweb, of a more traditional mechanical solution a pair of vice grips ( mole grips ) clamped on the oil feed line. .. so that it interferes with the kick-start arm.
or take a standard high temp ball valve and add a micro switch to the outside, for which a bracket would have to be fabricated. Some cam action of the valve closing would activate the switch. this would act as a an extra hidden kill switch, but add no active solenoid load. theres are pictures of a number of these Homebrew Solutions also on the web Google. .. BSA anti wet sump valve images. .. to see what others have done .
Any device fitted to a critical system should fail to a safe condition, a solenoid which needs power to be open does not fulfill a "fail; safe " analyses. In the case of a fuel solenoid , if the solenoid fails the engine shuts down , safe. If fitted to the lub oil system and it fails, the engine will be wrecked as it spins down. Simple is best, a 90 degree valve in the line which has the handle arranged so that the kickstart lever pushes it open is nearly foolproof. ( barring a bump start).
Wet sumping happens, even with a freshened up iron pump and a dressed up NRV my A65 will drop a little into the sump over a week or two, not enough to drain the sump for, it pumps back quickly. otherwise drain at the sump plug, and top up the sump with a few squirts in the timing plug hole. The new pumps from SRM are made to tighter tolerances , less oil will leak out when standing for idle periods. I havent tried one , but I see that as a big plus point.
71 Devimead A65 750 56 Norbsa 68 Longstroke A65 Cagiva Raptor 650 MZ TS 250 The poster formerly known as Pod
you can get an after market sump plate that has a drain bung(plug) in it as Gavin mentioned ............not ideal but it doesnt take long to undo the bung and drain out the bleed down oil and you dont have to take the plate off to do it ...........not sure where you buy them but someone here will know for sure.
my A65 can sit for about 4 weeks before it sumps to the extent that it smokes on start up so it is possible to improve the situation., but it does have a cast iron pump.
"There's the way it ought to be and there's the way it is" (Sgt Barnes)
Peter, I think you misconstrued my suggestion. A manual valve in the feed line with an attached electrical switch that is wired in parallel to the horn switch. If the valve is not turned On the horn will sound when the ignition is turned On. The valve can be left On except when leaving the bike for weeks. Gavin, your fuel "fail safe" case might not be safe. What if it fails, shuts off the fuel just before you enter a highway with traffic approaching? Engine dies and you are in the way of speeding vehicles. You certainly would not want to clamp an oil line with mole grips. I might cause the inner case to crack and block the line.
Dead easy solution. Fit an extnded sump with a magnetic drain plug. Remove the drin plug & put it on the fuel cap Or just drain & change the oil every time you expect to leave the bike unridden for more than 2 weeks. Oil is the cheapest part you can put into an engine.
Velos have an inline automatic valve in the oil feed line screwed into the bottom of the tank. They work on the same principle as the in line valves that every one says will stay closed and result in dire consequences. At this point, I have been running my Velo for almost 30 years and the valve has worked fine. Accordingly, I have used aftermarket versions on some other bikes with no ill effects. A couple of notes, however: Velo makes a big deal about making sure that the oil line is primed between the valve and the pump. There was nothing in the aftermarket valve instructions about that. The Velo valve is perfectly vertical, so that all of the force created by the oil level in the tank is acting against the spring. Some other bikes have horizontal or almost horizontal oil lines. Some years ago, when I got my A65 going, it would wet sump pretty quickly despite having had an overhaul and a cast iron case from Ed V. I put in an aftermarket valve, but made sure the oil line was primed and the valve was vertical. I have had no problems. My Norton Atlas started wet sumping, I did the repairs suggested, they helped for a short time. The Atlas has oil lines that are much closer to being horizontal. For a while, I simply crimped the lines with a pair of vise grips with duct tape on the ends. The vise grips stuck out far enough so that you couldn't use the kickstarter without removing it. Subsequently, I installed a metal petcock in the line. I ran a wire from the kill switch connection on the magneto to the petcock. The petcock is positioned so that the handle touches the engine case in the closed position. The result is that the magneto won't fire when the petcock is closed. I also took care to make sure that the handle was up in the closed position so that vibration couldn't close the tap while the bike was running. Another ploy, that I have seen, but not used is to use a petcock in the line, and relocate the ignition switch to a position where the key cannot be inserted without opening the petcock. If you didn't want to relocate the ignition switch, you could probably do something similar with an additional toggle switch in an ignition power wire.
The AJS/Matchless Owners Club in UK used to sell a valve with an integral microswitch. There were two versions --- one for magnetos where the off position of the valve earthed the magneto and one where the off position of the valve breaks the circuit --for coil ignition. HTH