The rent for the weapon factory is $50,000, which is fairly high because of its popularity.
In order to operate the machines inside the weapon factory,you must own/co-own robco (your name on the door(s)). Once you have been added to the door you can start doing the oils! First you press USE (default "E") on the button beside the oil spawner, then carry it to the oil holding chambers. Once you have all 3 chambers loaded you can press USE on the control panel, and you're done!