I had an 84 Pace Arrow Motorhome that had pretty bad detnotation under load. The previous owner used a 50/50 mix of 87 & 89 to solve the problem.
I did the same thing by using the octane boost product "Snap Octane Booster" at the same ratio saving 2.5 cents a gallon. One bottle of the Snap cost $1.00 and would treat 20 gallons or 5 cents a gallon.
To get to the 91 octane level it appears one would have, for MagnaCharger powered SSR's, to go two grades up as the standard for one grade up appears to be: "
Midgrade Gasoline: Gasoline having an antiknock index, i.e., octane rating, greater than or equal to 88 and less than or equal to 90. Note: Octane requirements may vary by altitude"
Link:
http://www.eia.doe.gov/oil_gas/petro...ne_grades.html
FWIW,
Harlan

04