It's the same basic process regardless of volume...oil/petroleum products are LNAPLs (light non-aqueous phase liquids) that will float (as they are less dense than water) on the groundwater table. The groundwater moves based on the gradient and this LNAPL will eventually find its way away from the source and potentially somewhere less than desirable like your shallow well. Yes, dumping a few gallons of used oil in your backyard trap like depicted in that old drawing is unlikely to cause any issues but why do that? Plenty of folks have had water quality issues form wells contaminated with by "old farmer John" dumping crap out back of his property.