A fuel expert once told me that the old fuel (if not varnished) can be burned, but will have lost it's octane rating, thus becoming prone to detonation. This is more so the case in warmer climates.
At all costs, we want to avoid detonation!
He suggested that the best bet is to add the Highest Octane rated fuel that you can find.
By adding the high octane, it brings up the rating of the old fuel to a more safe level.
45 + 45 = 90 gallons of fuel.... it won't take that long to burn through it.
At half way through it...... add more High Octane fuel, and you will have reduced the risk even further.
If concerned, reduce your Total Advance timing a bit until you've burned through this old fuel!
I can't speak about the so called "octane boosters", but I suppose that would not hurt either.
