After learning a certain oil change facility does not always change the oil filter when doing an oil change I went back to doing it myself. Things have changed on the motor oil shelf. So many choice...
You can mix synthetic oil with conventional. (See Synthetic Blend oil below).
Blended oil is marketed as Synthetic Blend which is both synthetic and conventional base oils mixed together, but there's no requirement for how much synthetic is used.
High Mileage oils typically have more seal conditioning additives to help prolong the life of the seals. I prefer not to use these oils unless you stick with them for the life of the engine.