***Probably the best scenario, BUT the reasons for not mixing weights are few, at best. Weights are viscosities, nothing more. If you are within the viscosity range for the temperature, and if your oil is clean and has the proper additives for the service you are running, there is no big deal.
I dont mix them, BUT my car requires 6 quarts, plus 3 ounces of oil. I keep a bottle of oil from which the 3 ounces can be made up. Then I add 6 quarts of oil per change, and top up with the odd bottle.
This is not rocket science.