Well, it depends if your car still has manufacturer's warranty.

You do not want to use mixed oils if it is still under warranty.

If you don't use your car very often, determining old change schedule solely based on miles is probably not a good idea. I would recommend that you change oil twice a year.

My car also takes 6QT synthetic oil. So, I bought two bottle of 5QT synthetic oil. I use one bottle plus 1/5th of the 2nd one.

Engine oils degenerates gradually. So, there is no apparent cut-off point. Changing oils after an engine run for 100 to 150 hours is a very good rule of thumb. If your engine runs for 150 hours, and suppose your car runs a zero mile, you still need to change oil.

Good luck!

Alex