It is not very common, but every now and then someone report passing stones on epsom salt, before oil.
It is not very common, because most people don't get diarrhea before drinking oil. If they would, maybe it would have been more common. There are also people who pass stones while doing coffee enema, no magnesium, no oil.
It just adds to the body of proof that Oil is not always necessary to get stones out.
All of those wild theories of oil being a factor in stones formation, just don't really hold water.
I wonder what kind of answer do those people have on your experience, those who are 100% convinced oil takes part in stones formation.