Gumout fuel system cleaner safe for manuals?

If the car isn’t running badly, I’d leave it alone. All you’re likely to do with Gumout (which IMHO is one of the least effective cleaners anyway) is stir up any gunk in there and end up with a chance of it plugging one or more of the small passages in the carburetor. If the car is running badly, you’re probably better off with a dose of Techron when your tank is down to about 1/4 or so, and if that doesn’t work, then resort to the spray. I would do an oil change too. as Researcher said.