The only time I have gotten extreme in my year of doing this was a completely frozen rusty gunky machine that had mud daubers nests in it from it's stay in the barn. I put the whole head in a 5 gallon jug of kerosene. The guy wanted me to take the treadle base too- and every drawer was packed full of wasp nests, just like the machine.
it did free up, and I made it a parts machine.