1993 Buick Century waste oil

They should be kept separate. Both can be recycled but they aren't made from the same base product. Oil is a petroleum product, and in most places that can be mixed with other petroleum products like transmission fluid and power steering fluid. Antifreeze is a glycol product and is essentially alcohol. I'm sure they have a way to get that out of oil that's being recycled, but they aren't intending to separate large amounts. They're trying to remove impurities, and those occur in small amounts.