Sounds like it's the oil pressure guage/sender fitting next to the back of the intake manifold. Have you checked that. Do yourself a favor and do not use the Nylon (plastic) type hose to the mechanical guage, It will eventually fail. Use copper tubing. I always make a about 1 1/2" loop in the copper tubing before going through the firewall to the guage. This allows for tubing to take up the flex and slight engine movement when using rubber engine mounts without damaging the copper tube. Also if you go to a NHRA dragstrip it is against the rules to have a plastic oil line, something you do not want going into the passenger compartment. I know I don't want hot oil spraying on my leg.