Ducky,
I have a 62 Impala with the same transmission. This is not the normal gear clunk. I can feel the gears spinning in the shifter and a little pressure from the shifter does not slow them down. The oil I used is something I am not sure about. I think is was what I had around for my 95 chevy truck..... may have been 80W-90W (was a black bottle and had a JT on it). Going to look at a new pressure plate next and compare with mine.
Thanks, Clyde