Stabilizer won't resurrect old gas, but it can slow the decay.

Adding fuel stabilizer to mixed gas that’s been sitting for a month won’t fix what’s already degrading, but it’s still worth doing if the gas doesn’t smell bad yet.

Gasoline starts oxidizing as soon as it’s exposed to air. After a month, especially with ethanol gas, some damage has already happened: varnish precursors, water absorption, maybe the start of phase separation. Stabilizer can’t undo any of that. It’s a preservative, not a repair.

What it can do is slow down further breakdown. So if the gas still smells like gas and looks clear, add the stabilizer, shake it well, and plan to use it within a few weeks. Don’t add stabilizer and then expect to store it for six months.

If it smells sour, looks cloudy, or has any separation, don’t bother. Dispose of it properly and mix fresh. That’s not where you save money.