Hi Phillip

Thanks for your inquiry, Industry standard for coolant is DO NOT MIX. Coolant manufacturers use different chemistry and if mixed can create Jell. we recommend a full system flush and use Radiator Flush https://penriteoil.com.au/products/radiator-flush Radiator Flush is designed to be used prior to new coolant being added to the vehicles cooling system as this will ensure the cooling system is free from rust, scale or any other residue which could block or foul the cooling system and prevent the new coolant from functioning as intended. It will also neutralise any leftover coolant remaining in the engine eliminating compatibility issues if changing coolant types.

Regards

Jamie