It may be a trail as suggested, but I've also seen water and oil seepage that seems to be a product of design. It is usually more of a water issue, but I've found coolant tracks between they layers of multi-layer gaskets, as if the fire ring sealed, but there was not enough clamping pressure to seal the water jackets. That could alse be happening with the oil. Putting silicon on the gasket face didn't help, because it was seeping between the layers. We always use some form of stop-leak in engines that I assemble with a layered gasket which usually stops the water seepage. If you are actually getting oil seepage from the gasket, you may need to get sealer in between the layers.