Symptoms of seized cylinder are, as you say hard to turn through and also poor/no idle, loss of power.
If you haven't already, strip down the saw to the point you can eliminate any other problems, like starter, coil dragging the flywheel, clutch problems, etc.
Turn the engine through by hand, plug out, and feel for tight spots.
You can also peek into the spark plug hole with a flashlight, preferably the "bore scope", light on a thin stem you can actually put inside the cylinder. Look on the exhaust side of the cylinder wall for black and blue vertical scrapes.
Also check for up and down play on the crank shaft. This can also cause binding due to crank bearing failure.
Obviously, none of this will go away by simply changing the fuel. These engines are rebuildable though.
Did 87 octane fuel cause this? I personally doubt it. Other factors, like running too lean, too much oil, not enough oil, packed up cooling fins, and just plain ol' worn out will do the same.