I read an article some years back on the oil - it said Pure Canola Oil. The article said to place the parts on a rack and tip the part into the oil when color was reached, IIRC.
Edit: It wasn't discussed here but on another forum. The short article/note concerned spring hardening/tempering. Brownell's included it in their catalog.
So the question of what oil remains...
BTW: There is an excellent 'sticky' on strawing parts here on the forum -
http://forum.lugerforum.com/showthread.php?t=18457