Should items like windshield washer fluid or oil purchased at WalMart be placed in the "auto" or "supplies/materials" category?

It depends on whose vehicle this is; typically, a Corporation might own a vehicle in its name and pays all operating costs. Otherwise, you described Personal, not business, if that is your vehicle.

Or;

Supplies would be if your business is Repairing and Maintaining vehicles and these are used for Customer's Sales or service.

Some perspective on the operations would be helpful. Thanks.