Low oil pressure / inconsitant oil pressure

The only part that might be a worry is the needle going into red at idle but an engine does not need much oil pressure at idle. If the oil light was coming on it would be of greater concern as that usually is set to come on at a lower level than is indicated by red on the gauge. If you really want some piece of mind consider adding a mechanical gauge. Since the shop said it was “operating normally” I would think they measured it with some sort of gauge and did not see a problem. You might ask them how they made that determination. If you were seeing low pressure at higher RPM’s it would be more of a concern.