NOT All pipe threads are tapered. There is a standard for straight pipe threads, NPS. I believe the oil pickup tube is NPS.

I ran into this pipe thread overseas. It seems to be more common over there. Sealing a straight thread pipe involves using sealer or something like hemp, turning the pipe into the fitting, then locking it down with a jam nut. This jam nut is supposed to put tension on the joint and get the threads to push against each other, creating a mechanical seal.

http://www.regalcuttingtools.com/products/taps/pipe-taps/straight-pipe-taps/nps-straight-pipe-taps

I would recommend Loctite 567 or 545. They both seal at higher temperatures, 300 and 400F and can be taken apart with hand tools.

R.