So, looking at your log and .cfg - I cannot really see anything wrong. The only thing that strikes me is that your reference velocity (ref_vel) is 0.1 m/s, which could be a tad-bit too low, but I assume that a draining fluid could accelerate to faster than that under free fall. Try setting the speed to 0.5 m/s and see what happens.
Additionally to the above conservative measure, you can try to ramp up the gravity a bit more slowly by setting the:
t_damp_bodyforce_end 0.1 ; (instead of 0.01)
in the domain parameters. That might help stabilize things a bit.
Finally, as a last resort, for these quasi-hydro-static type of problems, using the RIEMANN interaction scheme would probably be beneficial as it is more diffusive and would certainly smooth things out.
In order to do so, you need to set in the simulation parameters:
and in the phase parameters for oil/air:
The run will definitely be slower with these conservative settings, but you should get quality results, assuming sufficient resolution and no flaws on the geometry side.
Hope this helps.