OK, I'm sure this question has been asked before, but searching the forum, I really couldn't find a good answer.
What lubricant should be used in the Model T rear end ?
Suppliers offer "140wt gear oil" or "600wt gear oil" for about half the price. I'm aware of the different systems for rating oil viscosity, one chart equates either one of these with 50wt motor oil !
The "Bible" isn't a lot of help here, it simply says: 623 "Place 1-1/2 pounds of high-grade gear compound into differential housing."
We know to stay away from EP oils, which will cause damage to bronze parts . . .
So, what to use ???
Non Ruxstell rear uses 600 wt oil
Found in an earlier discussion:
140W for standard axles
90W for Ruckstells
The 1 1/2 pounds of high grade gear compound is a non-flowing grease. The early Ruckstells advertised that no changes were required when adding an under drive axle, the shift component was required to be packed with grease. Early Western Supply catalogs advertised grease for differentials. Some early parts books refer to the fill plug as grease plug and some call it an oil plug. They were never intended to be filled level with oil, just 1 1/2 pounds of grease. Many ended up filled with oil because it leaked past the rear seal and filled the housing. Many, when disassembled were filled with, grease, oil and water. There is nothing wrong with modern technology attempting to improve on the lubricating system of a model T differential however there are still some issues unresolved. Almost every month there is somebody reporting a leaking housing destroying the brakes.