Which side of the regulator are you considering the "back"? The side behind the knob, or where it attaches to the tank? If you've got water/oil leaking on the floor, check the bubble counter and hoses for cracks or loose attachments. There's really no conceivable way that your solenoid can have a large enough leak in it to pass oil without being completely obvious when the gas is turned on. Chances are it's just the lowest point in the system and the fluid is just working it's way there by gravity? Also, was this "freshly filled" 20# tank still cold when you hooked it up? Those things are frigid after a fill, and can take quite a while to warm to room temperature. This would raise the pressure significantly from the time it's filled to the point it reaches a balance...
I'm calling the back the opposite of the knob. Also, it is the lowest point, so its probably me failing with check valves.
Also, the tank had been filled and then sat for a few days so that it wasn't super cold anymore when I did all this.