STP is a viscosity increaser - it makes lubricants thicker. Why not use a more viscous lubricant to start with?

There are widely different opinions about which lubricants are best in vintage cars. The 600W lubricant is a steam oil cylinder oil (yes, for steam engines). The 600 value is not the viscosity, but the flash point of the oil. While it is very thick at room temperature, it thins out greatly in use as it heats. Many knowlegable Model T hobbiests use modern gear lube- 140 wt in regular Ford differentials and 90 wt in Ruckstell 2-speed axles. Gear lubricants are usually found in multiweight formulations (90-140) which would serve both uses. I understand the gear lubes can be found in single weight formulation, but not at your local auto supply store. One more item of disagreemenet is the use rating shown on the container. GL5 rated lubricant is for modern cars and contains additives that can be harmful to brass or bronze. GL4 rated lubricant does not contain these additives. How harmful is the GL5? One report I saw, by a person in the petroleum industry, thought it would be decades to cause appreciable damage. As I said, opinions vary. The final choice is yours.