There would be two likely sources. One is the rear seal on the crankshaft. The original was the "rope" type which literally is a rope made of fabric. Over time they will tend to dry out and drip oil.
The second place would be the rear gasket of the oil pan. It's just as likely to leak. It's the easier of the two to change. The crank seal you need to pull the transmission out and remove the clutch and flywheel and lower the oil pan.
When you do that one you should do the oil pan gaskets also.
There is a third possibility but low probability of the oil galley plugs leaking. Those are under the flywheel in the block and last but not least, the rear of the intake manifold could be leaking and running down the back of the block.
At some point you do need to address this since the leaking oil will get on the clutch itself. Some clutch materials are more resistant to the oil, others like organic linings won't tolerate the oil for ever.