I don't have long term data but I have been burning 91 to 93 "boat gas" in my IO 360 for the last 100 hours. I ran it in one tank and 100LL in the other tank during the test. Switching back and fourth between types, under all conditions, I couldn't detect any changes while recording all engine data and having it reviewed by an engine guru. I would have thought there would have been some measure that was different but no.
I even ran the engine in the dangerous "red box" area of operation and there was no difference. The lower octane didn't cause any temp spikes or detonation indicators.
The only detectable difference I could find is harder hot starts, mostly on hot days. Other than that, absolutely no difference other than about $2/gal. If I burn 8 gal/hr, that saves me $16/hr. If I run it for 1000 hrs, I save enough to rebuild the engine if needed.
Please don't take any of this as advice or gospel. It is just the way I choose to go. Do your research and make your own choices.
Data:
ECI IO 360 built from kit
8.5:1 pistons
Dual P mags
Silver Hawk Fuel injection
Cold air induction
Normal run 50LOP, WOT, 2350 RPM
Final note, not that it matters but there are planes at our airport that have never tasted lead and they are running fine, one has about 1700 hrs with no issues ever. Owner plans to run it as long as possible and only rebuild on condition, as needed, not per TBO. There are also a couple of Very expensive complex birds on the fields that have been run by the book and have both just had top ends rebuilt. Go figure!