600W Oil was used in Model A transmissions, differentials and steering boxes when they were first manufactured. No doubt that in their early years the proper lubrication was used. As they grew older and limited availability of 600W became prevalent "just any ole oil will do" was the rule of thumb. As time went by the owners paid little attention to the puddles of grease that became a "standard" for the Model A. If it wasn't leaking it must be empty. Or, if it ain't leaking something is stopped up!
As the cars became objects of restoration, investigating the lubricating products revealed that the wrong grade of oil was being used. In some cases the steering boxes were lubricated with chassis grease and this was a death sentence. The transmission and differential was filled with readily available 90 wt (Hypoid 90). That was because more modern cars and trucks used it. If it's new it must be
better! The end result for the Model A was that the oil leaked out of the transmission. Differentials were not quite as bad but they would also leak.
600W was manufactured and distributed by suppliers for Model A's for a number of years. Then the product changed. More recently the oil marketed as 600W is nowhere near the original viscosity. While we are here, 600W is not the viscosity characteristic of the oil. It is the name Ford gave it. It could have been called Micro Lube or some other exotic name. The viscosity was approximately 240wt.
Suppliers today offer oil that is called 600W. However, it doesn't come close to 240wt oil. So, the Model A enthusiast is faced with the problem of finding the correct lubricant. Here is what I found while searching the restoration archives.
Combining the 600W oil sold by suppliers with STP motor oil additive on a 50/50 basis can produce a reasonable substitute for 600W oil. Another method is to mix 2 quarts of 140wt oil to 1 tub of heavy duty wheel bearing grease and one can of STP motor oil treatment.
So, get the wife's mixer and make up a batch of the good stuff!