Can 20 gallons of diesel fuel mixed with 5 gallons of unleaded gasoline be filtered or treated so that it can be used in a diesel engine?

No, mixing diesel fuel with unleaded gasoline is not advisable for use in a diesel engine. Diesel engines are designed to run on diesel fuel, which has different properties and combustion characteristics than gasoline. Filtering or treating the mixture would not effectively separate the two fuels or make the gasoline safe for diesel engine use. Using this mixture could lead to engine damage and performance issues.