I'm sure this probably happens to many Omega watches of the same period, but it seems to crop up in F300 fairly often...and it's one of the worst things to deal with.

Over thirty years, the case back rubber gasket turns to a nasty sticky treacle. The only way to remove it is to strip the watch down to the component pieces and hand clean each metal part with hairspring degreaser and a cotton bud! If the goo gets onto the coils units, then you need to be extra careful as the solvent dissolves the plastic body of these units. In really bad cases, the goo sometimes finds its way onto the front of the dial....

Cleaning this stuff up is one of my least favorite jobs!