Mystery solved, sorta. I went to my local dealership and had the parts guy pull up the fuel delivery schematic on his computer screen. The top of this “drain tube” is plumbed into the neck of the fuel filler tube close to the gas door. It showed the line extending down, like in the picture I posted, but it’s not tied into anything. The parts guy had no idea what function this line served and I can only assume it might have something to do with the vacuum seal for the gas door???
Concerning the “oil”; the day before noticing the leak I poured some Lucas Oil injector cleaner in the filler tube which is clear and somewhat oily. I suppose it never reached the tank but got into this mysterious “drain tube”.
Btw, the overflow hole behind the gas door just dumps any water, gas spill, etc. on top of the inner quarter panel liner.
Thanks for everyone’s input.

If you want to add anything to the fuel tank you have to use the funnel found under the spare tire cover. If it's not there you can make a funnel from a bottle of Techron fuel additive. After it's empty just cut away the bottom of the bottle, makes an awesome funnel, easier to use than the overpriced thing they furnish.

Yes, my home made funnel was in my other car. Thanks