Mystery Oil Light

Evening chaps,
I drained the oil from the bike (1979 KZ) from the sump plug while I took off the left cover of the engine to give the clutch actuator some love.
I didn’t disturb anything else (I promise).
I took a 4ltr container of 10/40 oil and put it in. The sight glass is covered but I figure it will take time drain down?
I switched on the ignition (green and red light on) then did a few short stabs of the starter expecting the red light to go out…. But it doesn’t.
Am I missing something maybe?
Its a shallow sump model so no oil level switch?
regards
Steve
 

I don't believe the 79's have a oil level switch - they have a oil pressure sender.

According to the Parts Manual, engines after engine serial # 016038 have the oil level switch. That doesn't mean to say it's not possible to switch out a late model oil pan for the earlier model pans and retrofit the wiring so the level switch could work.

Link to Partzilla Parts Manual reference 1979 KZ1300 Oil Pan

According to the Parts Manual, engines after engine serial # 016038 have the oil level switch. That doesn't mean to say it's not possible to switch out a late model oil pan for the earlier model pans and retrofit the wiring so the level switch could work. 
Also note item #35 and item #2 which are the connector and pigtail wire for the level switch located on the back side of the oil pan of the late model oil pans (post serial #016038)

Link to Partzilla Parts Manual reference 1979 KZ1300 Oil Pan  (Item # 25)

Hmmm,
my bike doesn’t have oil level switch etc so I can discount that.
So the oil light is on all the time, which means it’s getting power.
I believe the earth is provided from the oil pressure switch when there is no pressure.
Am I correct in thinking that the oil pressure switch goes open circuit when pressure is achieved, thus robbing the bulb of an earth, making the light go out?

Am I correct in thinking that the oil pressure switch goes open circuit when pressure is achieved, thus robbing the bulb of an earth, making the light go out?

Correct.
Small test- Pull the pigtail off of the oil pressure switch and the light should go out. If so, you're problem is either a stuck oil pressure switch or a pinched feed wire for the oil pressure switch to ground. if the light goes out, then your next step is to confirm oil pressure by removing the oil pressure switch and installing a pressure gauge.