I know mercury states 89.
How many people getting by running 89 octane fuel in stock 700s? I have been running 93 as a precaution but maybe it is a waste of money.

Run Rec 90 if you can find it. I have found 93 blacks the transom up worse.

Ours are 91 octane requirement. 93 does seem to soot up the transom. I called Mercury and they said 93 doesn't cause it. I also asked about reflashing ECUs to 89 octane, but they said they hadn't done it in many years and really didn't know if they had the equipment to do it.

Octane is a measure of a fuel to resist knock. Boosted applications have a higher chance of knocking. I would say running 89 you have a higher chance of inducing knock/detonation and tearing up pistons. I would not recommend.

I have always run 93 I just looked at the mercury manual on I thought it said 89 but I could have been mistaken

I've run 89 many times without any issues. If motors are stock you will be fine.

Does it say on the sticker on the engine ?

I would just 89. I Believe the way that ignition works is it rolls the timing around until it finds a knock and adjust from there. If you're running higher octane it will not find a knock and back up the timing. I'm not 100% positive on this. This is only what I've heard from playing around with the ECM's

I run 93 in mine, no black on the transom.

.25 cents per gallon more for safety or 25000 more for a rebuild if your wrong. Only the guy filling the tank can make that call