Four strokes don't care about being inverted. I know what you are saying though. I've got 2 inverted four strokes, and when I check the valves, the pushrod housing and the valve covers are completely full of oil. I can just set the airplane upside down on a stand and let it sit for about 10 minutes before I check the valve clearances, or I can just be expecting all that oil and put some rags down before I begin. It won't hurt it. Actually, it probably helps it some since it gives the valves and the pushrods some extra lubrication. They get a little oil in there right side up anyway.