Good chance that you've found the culprit. Plastic lines are actually nylon, but they can get damaged. Suggest you unscrew the brass fitting that holds the line to the fitting in the block. Check the line for any damage, especially where the little copper ferrule is squeezed on to the line. Even if you see nothing, pull about an inch of the line through the brass nut. Lay it on a hard flat surface and cut it off with a sharp razorblade. DO NOT use a knife or side cutters as you may damage the end of the line. Slide another ferrule on the line and put it back together. Make sure you push down a little on the line so it bottoms in the fitting as you tighten. Don't over tighten or you run the risk of collapsing the ferrule and cutting the line or simply distorting it enough to leak again. I don' know how strong you are, but use the "two finger" approach to tightening. (that is pull in the wrench with two fingers.) Start the engine after completely cleaning all
oil off and from around the fitting and see if it leaks. If you see a little
oil appear, snug it up a little more.
The only other
oil leaks you can get from the top side is either valve covers or intake manifold and those you can usually spot pretty easily.