Right, first the oil is a SAE30 engine oil. Good job you were aware of procedure for removing the bus bar spring, because it should never be removed under compression, could be lethal if the bolts had run out of thread.
You might have got a "false neutral" and now when you put it back, with something like 3rd gear selected it may be alright. You say there is a lot of sludge and moisture, it makes me wonder if the band linings are breaking up or worn. Have you removed the tin cover on the top of the box? If so you will see the five band adjusters at the top, see how far down from the top face of the nut, that the male thread is. Better still, post some photos of these adjusters and bands.