I had a A7 with an engine that was badly damaged by the previous owner fitting a ball valve in the supply, which then stuck & starved the engine of oil. Fortunately repaired by the previous owner. A factor that owners forget is that when starting, they, quite rightly check the return flow. BUT, to begin with, that flow is only what has collected in the crankcase. It does not mean that oil is being pumped into the engine. One must continue to check until spurts of oil continue from the return pipe for some while to ensure oil is being pumped into the engine & scavenged back.
As has been said before:- Ball Valve- No, No, No, Sump Drain Plug- Yes, Yes, Yes. Get on your knees & worship. If you are married, you will be used to it!