Its the crankcase breather tube. It's wide open at the end - it is a little odd that any oil would bother to leak out as there isn't any pressure behind it. You could pull the tube out (one bolt at the top into the block, and I think one of the oilpan bolts) and ensure that it isn't blocked somehow (mud dauber wasp nest?) and clean the block and tube interface very well, and use some silicone sealer and reassemble.

Although it may be obviously coming from the breather tube, the oil might also trickle out somewhere else, and just collect there? The valve covers can be troublesome after decades of people overtightening the screws and warping the sheetmetal.