93 octane. There is a computer program to run 89 but it really limits the horsepower. If you take care of them the bottom ends go 300hrs and the valve train 150hrs(springs, rockers, lifters, guides, valves). oil change every 20-25hrs. and check the valve lash. Have the fuel injectors cleaned/tested every winter. they are an excellent engine

Much less. They will go 200 + hours before a top end refresh is needed.

How much power will it cost to run on 89? Are there other benefits to running 89 besides fuel availability? Does the motor get better running, longer lasting, less temperamental?

They "mercury" call it a 1025 hp tune for 89 but its closer to the 850-900 range. Uses more fuel. As far as longer lasting? everything is cycle time, how many rpm for how many hours regardless. My friends OL w 1075 has two computers on the engine. one for 89 the other for 93 just in case he cant get premium. There is a big difference in acceleration and top end mph.

I went 285 hours before top end refresh. A leakdown showed it was time.

They will last as long as your wallet can pay to have them refreshed! What would scare me about owning a pair is that Mercury abandoned the platform 5-6 years ago in favor of turbo motors. At some point the parts will become hard to source and the motors are not going to be worth much so you will hit that question of how much to spend before you give up on them in favor of something newer.

A service manager of a platinum Mercury service center once told me: "these are basically race motors, guys who buy them don't realize that most race motors need to be serviced frequently"