Stabilizer prevents bad gas. It won't resurrect it.

Mixing old and new stabilizer is a non-issue—the real question is whether that gas is still gas, and no amount of fresh stabilizer will fix fuel that’s already turned.

Here’s the thing: stabilizers are antioxidants. They stop gas from oxidizing before it goes bad. They don’t reverse oxidation after the fact. Once gasoline has degraded into varnish and gum, you’re not looking at fuel anymore. You’re looking at paint thinner with a gasoline smell.

So do the sniff-and-look test first. Siphon a bit into a glass jar. If it still smells like gasoline, looks clear, and has no water beads sitting at the bottom, you’re fine. Top that tank off with fresh fuel, add fresh stabilizer for the total volume, and run it. The old stabilizer from last year is probably spent, so don’t count on it. If the gas is cloudy, dark, smells like lacquer, or has a water layer, don’t put it in anything you care about. Ethanol gas absorbs water over the winter, and once it phase-separates, stabilizer can’t undo that either.

Borderline gas can be diluted and burned—one part suspect to four parts fresh in something you run through quickly—but that’s a judgment call. If it smells sour or like varnish, just dispose of it properly and skip the headache. Also don’t double-dose the new stabilizer on top of whatever’s in there. Follow the label for total tank volume. More isn’t better.

If it smells like paint thinner, it already is paint thinner.