Oil Light Popping On

My 72 Dart Swinger w/318 now has the oil light popping on. While driving no light, but as soon as I come below 10mph the light pops on. Once I start driving again light goes off, or if at a stop when put into N the light goes off. Oil level is good, and I recently changed the oil. I just put on 340 manifolds and had the exhaust installed today and thats when it was noticed. But car is new to me so haven't had much road time till today.

Thanks,