The way the coolant (oil) settles in the can is obviously designed to be mounted vertically to xompletely civer the coils. You never see a transformer on its side on a pole right? They are the same functional design. Now to mount a coil horizontally probably won't kill it but the design with the cap under the oil level is not the best orientation although millions of cars ran it like that. I think I've had 1 coil leak on me and that was after it got really hot due to a points issue.
