[Viv W]

Water works better than any penetrating oil, put the Tranny in a drum of water and leave it to soak , 3 to 4 weeks if rust is not too bad, but up to 3 months if really rusted solid. The rust is caused by water and water will dissolve the rust and penetrate better than anything else on earth. I have used this method on several motors, the latest was a Model A that had sat in the rain for 10 years with no spark plugs in the head, it took 6 weeks and some gentle persuasion with a 4lb hammer and block of wood. I got all the pistons and valve train out and even got the rings off the pistons without breaking a single ring.
If you don't believe me, try it .