As others have indicated, it will quite likely sprout again from the
stump. Engine oil or even better diesel will kill the stump and help it
to eventually rot.

Oil will act as a preservative.


Does this even work for ash trees? I am trying to get rid of the
stump of a tenacious one.

You need to get the poison into the roots.

Drill a number of holes about six inches deep around the edge of the stump.
Pour SBK brushwood killer in the holes.


What is the effect of using the oil/diesel in the deadly mix?

Simply a way to distribute the agent readily. Not very effective because it
needs to get into the sytem of the tree not just the surface