FWIW I have never had Coleman fuel (AKA "white gas") go bad or develop varnish or the horrid stink that old, regular, gasoline can and has done. I don't store Coleman fuel in the device (stove, lantern or heater) but fuel left in its original square can has lasted for DECADES and still burned reliably. I drain the devices before storing and always fill using a Coleman funnel that has a felt filter.
OTOH I have had some motorcycle tanks and carbs ruined from long-term storing of untreated gasoline left in the tank/fuel lines.
And for those who filter-brew their coffee: does anybody else use that trick of folding a regular "basket" type paper filter to fit in a tapered cone? It's handy if you're ever stuck out in the Tules (hinterlands) with all your paper cone filters used up and can still find that basket style anywhere...even a motel room.
I'll show you how if you need to know....