600W Oil was used in Model A transmissions, differentials and steering boxes when they were first manufactured. Must have done its job as so many Model A’s have survived til today. Later on it became harder to find 600W oil. Many used any oil they could find. People did often notice the puddles underneath their cars they thought was normal for a Model A. The saying went “If it wasn’t leaking it must be empty.”
As Model A’s started to go through restoration, it became apparent that the wrong grade of oil was being used. Transmissions and rear differentials were using 90 weight. People were putting grease in the steering boxes. These oils and greases were used in more modern cars. It was assumed new oils meant they were better. EVen for a Model A. Leaks were became common.
Different companies manufactured 600W for Ford and other companies. If you currently purchase 600W from various Model A supply houses you will notice it can be greatly diferent in many ways. None seem to be the same as the original 600W. A misconception is 600W is the weight or viscosity but this is untrue. 600W was originally about 240 weight oil. 600W is simply the name Ford gave to it so you could only purchase it from your Ford dealer. Unfortunatly current suppliers have no restriction on what they can call 600W.
There are a number of people and books that have tricks for matching the original type of oil. This includes STP oil treatment with a supplier’s version of 600W. Some also warn against these tricks. If you ask other Model A enthusiasts you will get even more suggestions.