I am using an ST3 on my TSIO-550. I considered the ST5, but, after talking with the folks at Skytek, chose the ST3. The ST3 is lighter and smaller, and, even though it sounds like ball bearings in a coffee grinder due to its dry gear design, has worked perfectly now for 360 hours with not a drop of oil and no slippage. It has plenty of power.
Note that you can’t just switch from an old Iskra starter to a skytek. The Iskra has likely worn the starter adapter to a point that the Skytek will start slipping either immediately or within a few hundred hours. This is not the fault of the Skytek. You should install a rebuilt adapter when switching to the Skytek. Not sure if this has anything to do with the oil leak, but it might be related.