Purely opinion but
I would suggest it comes down to quantity and location, the occasional litre or two would be unlikely to cause a serious issue, as soil bacterial would likely deal with the base oil.
As long as the oil was held in the soil and not washed into a water course and the soil had a high organic percentage, meaning sand/silt heavy soils weren't used, allowing time for the bacteria to work
Dumping large quantities could (?) Overwhelm the bacteria
But its not just about the base oil, modern oils contain all sorts of additives and waste oil contains all sorts of unpleasantness and even if these compounds are broken down, what is left behind?