Oil pressure light when idle?

A loose connection will cause the light to never come on, so that is not the issue.

A good /6 won't let the light some on at even a very low idle. The switch should come on and light the oil light at 8 psi or less. With a good pump, this should not happen unless the bearings are too loose. So it is either loose bearings, a loose pump, a pressure relief valve not seating as mentioned, a bad switch, or a bad/plugged oil filter. Put on the oil pressure gauge as suggested. Also, speak to the builder and ask:
- Did they install a new oil pump?
- Did they turn the crank? What did they set the bearing clearances to, for both mains and rods?
- Have you changed the oil and filter since it was rebuilt?