My experience is that you can run 87 PON fuel in 103 engines, especially in cool weather, but you may have to change your driving habits.

Detonation is most prevalent at low revs, so you need to feed the throttle slowly. When your tank gets near empty try a few gallons of 87. TURN OFF THE RADIO/STEREO and listen for detonation. If you hear it, learn to drive around it. If it detonates significantly and you can't drive around it, fill it up with 91. You're stuck with it.

Hot summer weather with the A/C on is going to make detonation more prevalent, so even if you can get away with 87 now, you may not be able to in the heat of summer.

Duke