I prefer to use brass freeze plugs- - - -they don't corrode away nearly as fast as steel ones. Yes, a little Super 300 works to seal the freeze plugs, too, and it works antwhere that a cork or paper gasket is needed. Greasy crud can be cleaned out of a cooling system by running a strong solution of Tridsodium Phosphate and water in the cooling system, getting the engine up to operating temperature for 30 minutes or so, draining and flushing with clean water. Follow up with a 50/50 antifreeze/water mix. TSP (concrete cleaner) can be purchased at virtually any big box hardware store, or on Ebay or most other online sources.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!