Ward: It's John Lutz. How are you? That is simply water mixed with the primary fluid. Damp environment where stored...pressure washing blasted water past gaskets/seals....water got in thru primary breather hose....water got into clutch cable and trickled down into primary....multitude of possibilities. Simply change it a few times with the least expensive 20W50 JASO-MA or MA-2 rated oil you can find and get as much heat into the motor/primary as possible. Should resolve it.
And if "rough running" as you mentioned, I'd install fresh plugs and carefully check the wires for age/vermin damage. And if sitting for several years in all likelihood you'll be battling fuel pump filter(2) problems so plan on servicing that as well.