What would be the purpose of installing an O-ring where Porsche deemed it not necessary? I'm sure they took the slight internal pump leak into consideration, so maybe installing the O-ring (if possible) could raise oil pressure more than optimal? Not that the engineers always get things right, but with so many years of development on this 9A1 engine, I'd guess that's not an issue at all. But yes, I've seen car manufacturers skimp on pennies before, so always possible. Will be following this thread out of engineering curiosity.