You need to clean the engine so that you can find exactly where the oil leak is coming from.

I would be willing to bet that it is coming from the tube that goes from the air cleaner oil separator to the oil pan. This is very typical with the non turbo 617's. The engine is getting a little tired.

You should indeed clean and look for EXACTLY where the leak is coming from. If you determine that it is where I think then you should do a compression check.

It is most likely due to worn liners, pistons and/or rings.

Good luck,