Wrong!
I've witnessed it firsthand more than once. The oil will definately leach, and will kill the soil down for quite a ways if let go. Turning it 6-7" deep as soon as possible (or as deep as needed to go to get below where the oil has leached), buries the contaminated soil before it can contaminate the soil farther below, and makes it easier to re-seed the spot. The only way it'll bring up weeds is if you leave the spot bare and weed seeds blow into the area and take root. Weeds just don't lie below the soil's surface just waiting for a chance to be brought to the surface to take hold. If the lawn has been properly fert'd regularly, there'll be no weed issue if either the soil is turned and re-seeded, or new dirt is thrown down and seeded as soon as the contamination is taken care of.
Mr RG,
I have to respectfully disagree...like totally.
For soil to become toxic it has to contain or become introduced with a certain level of hazardous component. Gear oil of this type and in this instance is so low in toxicity it would not nor could not warrant removal of soil.
I would akin this "leak" to the same amount of hydrocarbon as does a chainsaw spewing bar oil as it's cutting wood. It's not operated through crankcase. It also is so low in quantity and high viscosity it'd have to be larger volume to impact and create any environmental hazard.
The only one big issue was hot liquid on contact.
The small issue was gear oil in minimal volume {rate}.
what's going to appear are small patch of dead grass and stream path,easily fixed with seeding.