the reason you want to mount a
oil filled coil vertical is the air bubble inside of the coil. Most if not all of the
oil filled coil have a air bubble. Probably each is a different size. I don't think they try to control the size of the air bubble when they manufacture the coil. But I don't know.
Went you lay the coil on it's side the air bubble exposes the wire coils. It may or may not burn the coil wire into. Some fail within a 100 miles others last for ever.
I don't take a chance so I mount all
oil filled coils vertical.
Just my experience
Dwight