Mixing brands is not a bad thing but they are definitely not all the same. There are a number of different oil chemistries that can meet the requirements to get an API license. As Jerry said, a mix is better than running low but if possible use the same brand to prevent unintended concequences of chemistries that do not blend well. Dollar General and other name brands are just that, brands. They have a sourcing agreement with an oil producer to provide a product with their name on it. Unfortunately you have no way of knowing if they changed sources so I prefer to stick to oil company brands.
