I had this problem on a late 80's 318. The problem turned out to be a leak at the rear of the intake manifold near the distributor. I took it off and used Permatex Right Stuff sealer at the front and rear joints. Good stuff.

Also check to make sure the valve covers are not being held up off the heads by an aftermarket intake. Mine were. My valve covers are the cast MP ones with a lip. Sometimes the stamped stock units are warped around the bolt holes and need a little hammer work to flatten out.

Now I am chasing an oil leak at the passenger front that is not the fuel pump, valve cover, timing cover or dipstick tube...