No oil pressure at idle

My 67 dart, 273, 150,000 miles on 1st rebuild 319,000 total miles,

Would turn on the 8 psi (or is it 11 psi???) Indicator switch when the engine was at operating temp on 10W-30

Now I run 10W-40 oil and don't have that problem any more
Scary thing is it didn't show any pressure at idle with the fresh (cold) oil either.