A oil seal
would seep and take a long time to loose even a pint of oil. If the oil filter
is tight and not leaking, the external steel
oil line behind it could be the primary cause. If it looks lumpy/rusty it probably has a pinhole leak in it.
has a copper line replacement. Or if you find 3/32" copper tubing, you can drill out the banjo bolt (with 3/32" drill bit) and rear 3 way block and solder in a new oil line. Solder, do not braze.
I rerouted mine to above and around the starter.
If the oil line is good, then check that the crankcase breather (left rear of valve cover) is not clogged.