My M3W tank oil level seems to be maintained for several weeks of non running but I don't think that a drop in oil level over several days is a problem. There is possibly a slow leak down through the oil pump into the sump, there doesn't appear to be a non return valve in the line to prevent this. As long as the oil level is checked a few minutes after running, that should give a true level. The oil in the sump will soon scavenge back into the tank after start.
