check this link:http://www.faxonautoliterature.com/1956-GMC-Truck-Twin-Hydra-Matic-Transmission-Original-Supplement-P15947.aspx , This is a book that should give the right information. Also it may be that if the clutch pack have been replaced with modern frictions it may well use later style fluids. Also try B&M transmissions in LA, phone 707-544-4761. I see there are several suggestions. and I can't remember what they used. All the people that I knew who knew are dead.

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