Hi Lozsing,
Filter bowl facing down is the correct procedure on all filters. It allows you to remove the bowl for cleaning with the least amount of spillage.
In the early days this was a 'problem' with Shurlo pumps. The pump was mostly fitted upright with the pump mechanism on top and the electric motor underneath. If you bought a shuflo screw on filter it would tighten and be leakproof in the standard recognised postion (bowl facing down).
Then Shurflo decided that the pump should be fitted the other way round, ie electric motor on top, and this effected the position of the filter.
We had a filter working on one of the vans for a couple of years up-side-down and worked totally satisfactorily, but eventually it bugged me so much I replaced it with filter that is tightened with a wingnut coupling so can be fitted in any position. Still keep the original as a spare.
(In very cold weather we empty the pump by unscrewing the filter bowl and pour the water out, switch the pump on to suck air into the pump mechanism. This will remove most of the water from the pump mechanism - any drops remaining can freeze without damaging the pump. This won't work if the pump is mounted very low. We also do have frost heaters in the vans, but this is an additional precaution, especially between Christmas and New Year when we don't work and generally don't pay too much attention to the vans either.)
Spruce