automotive lubricants (ATF, manual trans fluid, or heavy motor oil) as bike chain lube?
In my experience, the objective of a good chain lub for a bike is to maximize lubrication time while minimizing dirt attraction.

That happens for three reasons:


The chain is totally exposed in a performance bike. You have dirt coming from the ground, the wheels, the air...
You legs are surprisingly close to the chain, and may rub it quite easily.
The situation where your chain snaps from the crankset and you may have to reinsert it manually in the crank teeth is not quite rare.


That is why there are a great variety of chain lubs. You can go from minimum dirt attraction (wax) to the maximum (wet).

I would say that if you are perfectly happy with a wet lub, a switch to an ATF one would not pose a big problem. What you do not want to you use are solvents, most notably WD-40, they do not act as a lube long enough.

Do not use thicker oils, as standard engine oil: they will render your chain a mess in not so long and they won't penetrate deep enough in the links.

On the other hand, if you want to minimize dirt attraction, something I prefer personally, I would use a bike chain specific dry lube or wax.

As a bottom line, you know that a 120mL bottle lasts like forever? You have to apply only one drop per link, meaning only 0.05mL per link.