Under NORMAL operating conditions there is ONLY two ways that a magneto can get wet from fuel/oil. The first being the easiest to fix but the most remote would be through the cover gasket...highly doubt this can happen. So this really leaves the source, and that is the seal behind the magneto (crankshaft seal) is allowing the fuel/oil mixture to seep. Now you say thick...yes.... when a fuel/oil mixture is deposited on a item and the fuel itself evaporates, a thick sticky residue (actually the oil) is left behind. There are two ways of fixing this problem, the first is to replace the seal, which I'd do, the other is to enclose the magneto in something...I'd suggest using a portion of a large zip loc bag. Remove the magneto and magnet, poke a hole in the bag and slide it over the shaft. Replace both magnet then the magneto and secure bag with the coverplate. Remove extra with a razor blade. This just might work.