Premium gas 91-93 burns more in a more controlled manner - 87 octane can detonate in the compression stroke of a piston which can cause engine knock and piston/cylinder damage. I don't know how the 85 351W controls engine timing. Newer cars will detect ping/engine knock via a sensor and adjust timing to eliminate ping/knock.
If you made it to 1960 hours what is making you want change the fuel octane? Higher octane fuels are for higher compression engines... hence racing fuel aviation fuel....
+1 on Cant' repeat's advice.
...A bad day water skiing still beats a good day at work...1995 Pro Star 205....