Stay away from anything containing more then 5% of ethanol, (so don't put anything labelled E10 or higher in your car) if you don't use it daily, the ethanol will turn the fuel bad in about 2 weeks.

As described before, part of the fuel will become a jelly like substance, and you can't just stir it back into the fuel.

Fuel not containing ethanol if it's not gone stale, eg, less then a few months, it should mix in again.

If you really never use the car, just top it off with aspen4/aspen+ (fooking expensive though), that shit never goes stale, but worth it for storing a vehicle over winter for instance.