I believe 600 weight oil, originally used in Steam engines are used in the transmission and differential. At least my manual says to use them. 30 weight non-detergent oil for the engine unless it's been rebuilt then a 30 weight detergent can be used from what I found out. You can get the 600 weight oil from Snyder's of Max Model T and Model A suppliers.
Where Are You From? Oviedo, Florida
Do You own a car built by Durant? Yes 1928 Model 65