Update...things that were tried
1)tried stuffing and wrapping sheath with tissue paper, tightly binding the whole in packing tape and put it in the oven with only the light on (maintains temperature of about 40*C) for two days, worked a little but not much
2)completely covered the sheath with a ton of baby powder and repeated the technique above. Worked much better this time as the talc was stuck onto the oil. After brushing, and wiping it down, it was evident that there was still quite a bit of oil to remove.
3)finally added a bit of dish-washing detergent to some lukewarm water in the sink and submerged the sheath and gently rubbed it with a rag. After two minutes the water became brownish-orange. I drained the sink and immediately repeated the operation two other times, for only a couple of minutes each. Then I patted it dry and hung it in the oven (just with the light on).
It now feels a bit dry, but I'll wait a few weeks to see if any more oil rises to the surface.
Anyways, so-far-so-good, I'm really happy with the results.
PS...although the baby powder was washed off, one can still smell it and the leather feels silky smooth, but perhaps that will dissipate in the next few weeks