When you take the filler cap off, you're creating a massive vacuum leak. That's normal for the engine to stall. If it doesn't stall, then you'd have to worry.
John 82ZXT

 

the l28 is equiped with an oil pressure safety switch. this helps by retarding the ignition when you loose oil press. it really helps when your driving down the road and you blow a seal and dump all your oil. it will kill the engine so you "shouldnt" seize your bearings. this does not work all the time. i know.