Almost no oil pressure help

i have had my Dart running for about 1 month with no engine problems and a total of 25 miles on it and since I bought this car from someone who had not ever had the engine running I decided to put a oil pressure gauge on it because the oil pressure light was on.....with the mechanical gauge installed... Upon starting the car... The gauge shows no pressure and after 2 laps around the block the most the gauge has jumped to was 20psi. Thoughts comments recommendations....