I have a 1987 Evinrude V4 120 hp. I was having a discussion with someone and they were telling me that it is not a good idea to run mid-grade gas through the motor. Is this true? also would it be safe to put carb. cleaner in it?

I'm not a tech but have always been told that those older motors should have premium in them.  My Optimax is the only motor I have ever had and run mid grade.

Acutally, you problaby would be better running the mid grade over the lower grade. The 87 motor was the last year they used the smaller chamber heads that boosted compression about 15 - 20psi. In 88 they went to a more open chamber head that dropped the compression down to approx 90psi to better handle the low octane fuels they were going to.

With fresh gas, you can run the low octane with no problems, a motor with 130 psi compression can run it but there is no margine of error. Gas that has set in the tank for a month or so can loose enough octane to cause damage to a motor with that much compression. I think your motor runs approx 115 psi compression, if in good shape, so you have a little margin of error, but if you you leave gas in your tank, use the mid grade. If it sits for a couple of months, add about as much premium to what you have in the tank to help replace some of the octane that has evaporated off. Gas looses octane rather quickly.

I'm not sure what you are refering to as carb cleaner.  If you're talking about the spray cans of carb cleaner you spray in the throats, don't waste you're money.    You can run SeaFoam in the tank, that will help.  You can also spray a decarbonating solution into the carbs by following the directions on the can.  Something like the BRP 777185 Engine Tuner.   This should be done approx every 50 hours.  This really helps a motor.

I run high octane in my '72 Johnson, I just made sure I get it at a station that has a high turn over rate for premium fuel, that expensive gas can sit in an underground tank for a while in these times. If you have a local recetrack, ask some drivers where they buy fuel.

As far as carb cleaner, if I own it, and it has a spark plug or glow plug, it gets seafoam.