1986 90 hp Merc - is 91 octane too high?

[SPOONMINNOW]

The only non-ethanol gas sold near me is 91 octane. Merc customer support said 91 is ok but never over 93. Anyone run a higher octane than in the owner's manual?

Anyone use an in-line oil/water separator?

[rustyself]

If Mercury says it's alright, it's alright. I don't think you'll find a higher authority than the manufacturer of the engine on an internet forum.
As far as separators go, it adds another failure point to the system.
Keep fresh, non ethanol gas in it and go have fun.

[SilverFox579]

From the FAQ in the 3L forum, recommendations apply to ALL engines, pay close attention to the last bullet point - Oil, Battery, Spark Plugs, Fuel and Fuel Additive Recommendations

Fuel Selection:

Make your fuel selections based on the following criteria, IN THE FOLLOWING ORDER:

-High Traffic, Top-Tier (Name Brand) Fuel
-Appropriate Octane Rating (see above info)
-Lowest possible ethanol content (NEVER to exceed 10%).
-While it may be preferable to utilize a fuel that contains lower (or no) ethanol, use of fuels that are SUBSTANTIALLY higher octane can result in decreased performance, decreased fuel economy, and potential for carbon buildup. Higher octane fuels burn SLOWER and COOLER.

As to whether to run an inline fuel/water separator, if your engine already has one, do not, as it can add a restriction. If you provide your engine serial number one of the pros can advise whether your specific engine should, or should not, already have adequate filtration.