Also being one who speaks of personal observation and experience, I can tell you that I have two samples of 3 in 1 oil under observation, one from around 1964 when under the Boyle Midway brand, and one recently purchased, under the ownership of WD40. These were the first two oils to cloud up white and jell up, in about 6 weeks. The ATF is completely fluid, no change in viscosity, the red color has faded to amber. The Marvel Mystery Oil is also now amber, the solvents have evaporated and the viscosity is about like 90 weight gear oil. As I tip the container I see areas of coagulation. I am not going to tell anyone what oil to use, but I will report personal observations, and not repeat gossip. If you must have the smell of Mystery oil, simply add a few drops of wintergreen to your favorite oil. ATF is probably way overboard for the lube needs of a sm. It is also the transfer case lube for many 4 wheel drive vehicles. Although thin, the EP (extreme pressure) additives protect the gears and bearings through more heat and pressure than a sm will ever see.
John in WI