Depends on the stove, but persistance can pay off with old fuel. My oldest remaining stove, a no-model-number Peak I (400 type), still has the original generator on it and it has burned quite a bit of nasty old fuel - the gen cruds up for a bit, but keep running it and it burns out the crud and behaves well. (Stinks badly on shut-down with old fuel tho - baked varnish smell) My original MSR XGK, almost as old, doesn't care - if I can light the fuel, it burns it. My even more ancient Svea 123 was the same way.

The difficulties with old fuel go up rapidly as the temps drop. What is a PITA at 50 deg-F is frustrating at 0 Deg-F and dang near impossible - depending on the stove - at -40 deg-F. (Actually, I found that locally refined Blazo was noticably better in extreme cold than Coleman Fuel when both were fresh. Crummy shelf life, compared to Coleman fuel, though. YMMV)

Fresh fuel is a LOT easier and nicer to use, though.

Having written that... these days we almost always use a PowerMax or MSR WindPro for backpacking trips of a week or less - PowerMax exclusively in cold weather. Big green box in base camp, of course - can't beat those. If we were back in the interior of Alaska, I'd stick to naptha stoves, of course.

Regards,

Tom (another old soldier)