Curious whether you can run a horizontal compressor in the vertical position? Is there a shorter life span because of the way the motor is situated?
As far as the compressor air tank goes, it doesn’t care the orientation. It will be difficult to drain water if the drain valve is not at the bottom, and if the air discharge port ends up being close to the bottom, you’ll be expelling water along with your air. These issues can be overcome.
Where I don’t think it’s a good idea to turn your compressor on it’s side, has to do with the motor.
If your compressor is oil lubricated, then turning the compressor on it’s side would be like turning your car on it’s side, and running the engine. The crud in the oil sump would quickly spread throughout the compressor pump and wreak havoc. Not to mention oil would be leaking out the fill port.
Even if your compressor is not oil lubed, I would expect that the orientation of the compressor head is correct as it’s designed, and in my opinion shouldn’t be changed.