[dcarrb]

I wouldn't leap to the head gasket conclusion absent other symptoms; that's your worst case scenario. Even if the stuff in the tank is stop-leak sludge, it could date back to something dumped-in years ago by a previous owner trying to limp around with a busted radiator. I'd just fill with fresh coolant and keep a keen eye on fluid levels and the temp gauge for awhile.

If you're having overheating troubles, coolant boiling in the overflow and such, that's a different story.

dcb