I'm pretty sure there is a fed regulation that every modern car made has to " run" on 87 regular. ( computer equiped cars with knock sensors not older non computer cars )

I ran my 97 SL320 M104 straight 6 on mid grade for a while then moved to 91, there was a difference in the sharpness of the engine when the switch was made.

If you drive on level ground at 40 ish MPH regular would probably be just fine.

Also see
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