Is steam oil proprietary or can a substitute be found in the automotive section?
Autos do not run on steam, so you can't find proper steam oil in an auto parts store. DO NOT use any auto oil in a steam engine lubricator.
It is a generic product, but only available from steam locomotive suppliers.
Accucraft sell it: AP28-203 for $5
Another source is The Train Dept:
I found a pint can and it has lasted me many, many years. I do have a small bottle for actually filling the lubricator so it gets refilled as needed from the can.