It's more or less a laxative, and it's claim is that it stimulates contractions which bring on labour. More often though it just causes the female to be extremely uncomfortable and remain pregnant.
The motor oil you're thinking of is Castro... I think.
Its a laxative... And back in the day people would use it when they were overdue to jump start labor...
I used it with my second child it worked for me labor,lasted maybe 3 hours no pushes and my daughter was born healthy