Why did plumber pour oil in sump water?
I've heard of using a layer of oil to prevent evaporation or discourage mosquitoes or things like that... but that requires levels be set so the oil stays above the pump inlet at all times, and it assumes that some loss of oil into the exhaust is OK (which is one reason it might need replacing).

If the town said no, I'd say it's a reasonable position for them to take, and the plumber should know the actual rules... so I think it's worth telling him that there was an objection and asking him to explain why he recommended it.