How far up on the lines does your factory oil pressure gauge read? The gauge's sender is near the oil filter on a spline that sticks out of the block. You could try an autometer mech or elec oil pressure gauge if you don;t have one already. You can mount the sender in the fitting the factory gauge was using or use a T fitting and run both gauges. That way, you kow you are getting an accurate reading.
The dash light could be indicating oil pressure being low, which sounds possible due to it coming on at start up when the pump is building pressure initially. It should not stay on for more than a second or so if all is working properly. How long does it stay on after start up?
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