Do spark plug tube leak oil?

Yes, they have O-rings at the top lip of the tubes.

I assume you are talking about a slant-6 motor.

Yeah, you can remove the spark plugs and get the spark plug tube O-rings at the auto parts store.
Usually order and have by the next day.

I replaced mine and also used some clear silicone gasket sealer around each O-ring for added assurance.

Alot of guys take off the spark plug washers if they are removable so they don't distort the spk plug tube.

There are a few articles on this in slant six.org website.