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600W cylinder oil is a spec that predates modern standards for viscosity. It is still sold more for it's capability to work around steam or for old style worm gear drives. It is a straight mineral type lube that is comparable to GL1 SAE 140 gear lubricants. Any GL1 SAE 140 type lube will work in a model A transmission. STP is a viscosity builder so a person should mix it with a gear lubricant such as SAE 80 or 90 in order to get an acceptable viscosity. Straight STP is more viscous than SAE 140 gear oil and may cause stiffness in the shifting of gears. Too heavy a viscosity is almost as bad as too thin a viscosity. Another thing to consider is if you look at the advertisement in post 2 you will notice that is now has ZDDP additive. This is an EP additive that may have a detrimental effect on the phosphor bronze or brass in the transmission. This is why they used straight mineral oil in the first place. STP is designed for internal combustion engines.