if you think it leaking between the threads on the oil filter mounting bolt take a filter and thread it on slowly after you get a few threads down try and move it up and down left and right(as you completely turn it on) if it moves too much you know the threads are shot. but like cheesemaker said its more than likely going to be the rubber ring on the surface of the filter or a bur or ruff spot on the machined surface where the filter seals on the block. emery cloth will clean up any burs that might be there.
