Now an interesting fact The Voyager crankcase has an oil passage already drilled, tapped and plugged in the location where the oil pressure switch is located on the KZ1300 crankcase, so an oil pressure switch could easily be incorporated on the Voyager.
Patzilla 1986 ZN1300 crankcase diagram item # 92066-059
The only hitch would be how to connect it. The ZN doesn't have a oil warning light rather it has an 'Alert' LED that flashed to indicate a problem and a 'display which' lights up (not really since it's a LCD) to display what the problem is. I haven't looked at the wiringe diagram recently so don't know if it's the ECU controlling which LCD alert is activated or a wiring/diode array. In any case it may be possible to just connect a oil pressure switch in parallel with the level sensor assuming the logic works the same way for both switches, i.e. switch open good, switch closed bad.
I agree that an oil pressure warning is an absolute must on the Z1300. That said, if cylinder, pistons and piston rings are in good condition and properly run in, those engines uses very little oil. I have owned two Z-1300 A-1 Neither of these were oil users. I never fill any oil up between oil changes on my present bike. Even if I am sure the small sump is quite adequate for normal use, I was so lucky to find a big sump on Ebay So together with the clutch cover with the large window I now has 6,2 litres oil capacity.
BTW remember if you change the small window clutch cower to the large window cover, you will also need the the baffle 18020-1020 to avoid oil foaming, as the large window is higher up..