My brother's mother-in-law has a St. Jude aortic valve that leaks like that, I believe. She's had the valve for 12 years, so far, with no related problems, and the leaking is the same as it was right after surgery. It seems unlikely to me that a mechanical valve could "develop" a leak like this over time. I think that all mechanical valves (including my On-X aortic valve) are designed to leak a little bit to purge any blood that might stay between the leaflets. I suppose some people's valves might leak more than others due to different blood pressure, EF, heart rhythm etc...
