I switched to synthetic oil to reap the benefits of extended oil change intervals. I change oil at 12K to 15K miles or once a year. I only use Amsoil, Royal Purple or Mobil-1 Extended for these extended periods. Is MMO good for these extended periods between oil changes?

Marvel Mystery has not been tested at extended oil intervals.  It is compatible with synthetics so that is not a problem.  If you are doing severe type of driving, i.e. short trips and lots of stop and go traffic, you may want to change your oil more often.  Short trips and stop and go driving use up the additive packages in the oil much faster and stead high speed highway driving.

My driving is almost all highway miles to work and back so that’s not a problem. Sounds like to be safe I should wait until about a month or two before my normal oil change time and then drain one quart of old oil and add a quart of MMO. This should give me around 2000 to 5000 miles to run the MMO and then change all the oil.