The fuel after a short time can congeal, (go thick) doesn't happen so much if its in a big container, but in the little small gaps in ur carburettor needles it goes thick and blocks the fuel getting through. If it were me, id make a note of the current needle settings, then carefully remove the needles, and clean out the holes with a solvent like brake clean or something similar. Give the needles a gentle rub with a clean cloth, Then refit and adjust the needles.

That opti fuel, I bought a gallon if that and ended up binning it, pure rubbish car would barely even start, when it did it wouldn't rev up. Other people tell me its fine etc but that was not my experience, i got proper fuel and car was running perfect again.

But yes if ur fuel is 25 years old u will doubtless need new stuff.

glowplug, I would imagine it should glow orange, certainly mine do, id imagine 1.2v or 1.5v should both be enough to light it. Perhaps if ur igniter is old though, that whilst it is charged up, its not providing enough current yo light the plug properly. Like when u charge up a flat car battery, it goes to 12v, and itll run a radio, but soon as u load it with turning over the engine it goes to 0v and wont work properly, could be the same story for ur glow igniter