OMG.
I hope she didn't know that's where her babies were. I mean I know there are worse places but that makes me want to puke.
It would certainly conceal the smell, yes. It would also make it very difficult to get trace evidence, because they'd have to rinse the oil off first! That would be extremely difficult to do without destroying any evidence. That 48 hour notice might not be an issue after all.
Gas would evaporate at least. So hopefully it was just gas, that is a stronger smell so maybe he went with that. Unbelievable. I cannot even imagine putting your own baby girl in a horrible lonely place like that and LEAVING. I couldn't do that with a stray kitten much less a human child.