Quaker state enhanced durability high mileage oil compatibility?

used quaker state enhanced durability high mileage oil 5w20(red jug) when i changed my oil and now i'm a litre short. no one in the small town has the same oil and need to know what i can mix with it. i wouild like to stay with the same kind of oil but cant figure out for sure if it's synthetic or a blend or just regular motor oil.