You have a mechanical gauge so you can ignore the above post about the electrical style. It doesn't need to be grounded or anything else because it works off direct pressure from the engine to the gauge. Just be sure the tube hasn't gotten kinked or come loose. Good oil pressure readings are about 10lbs per 1000 rpm. Your engine will stay together with 10lbs at idle in gear and the 35lbs you had were still ok to drive.