Get rid of the 90W "modern" gear oil and go get the " 90 WT mineral gear oil" as orginally recommended in the manual . Tractor supply has it as Traveller Brand Ford Tractor Transmission Fluid Mineral Gear Oil. The word is that the modern oils are too slippery and do not allow the right amount of friction for the synchro's to work correctly. Thats the brand I run in my SM420 in my 1954 3600.

Ron - - Dusty53
1954 Chevy 3604
In the Gallery Forum
"You can't dance with the Devil and then wonder why you're still in Hell."
"They will forget what you've said, and they will forget what you have done but they will never forget the way you made them feel."