Can a city all of a sudden impose certain zoning laws

This answer is for informational purposes only, does not create an attorney-client relationship, and please note that no specific research has been performed and the law in your jurisdiction may vary. I recommend that you hire an attorney.
As I understand your question, the town has not attempted to zone your land, but has decided to limit the type of vehicles which may travel on the road whereby you access your property. I do not see this as a clear-cut prior non-conforming use case. They don't seem to be directly stopping you from doing the oil changes, but your problem is that you access your property with what sound like commercial vehicles, and the town wants to forbid those for your street.
Traffic signs alert motorists to underlying ordinances. Find out the number and exact wording of the underlying ordinance. You may be able to challenge it as over-broad if it refuses to make exceptions for local access. You could also perhaps challenge it as "ultra vires" pseudo-zoning, since, in effect, it prevents commercial use of any properties along that stretch of road, but without going through whatever process North Dakota mandates for proper zoning ordinances.