For the latest update on the ST 51 on my SuperSportster, it is running absolutely flawless. I did try some Mobil 2T full synthetic and I won't be running that anymore. It leaves a very hard black carbon on the tip of the plug. The carbon is almost as hard as glass. The top of the piston was as clean as brand new before the Mobil and now it has three areas that are black with carbon as well. Back to the Pennz/Zenoah oils for me. Either works very good in my engine and with either one the piston was staying as clean as new. It now has about 5 hrs on it and runs just as strong as ever. No unusual noises or anything. I don't think I will tear it down until I see something going wrong. I kinda believe in the old saying "If it ain't broke don't fix it."

Edited to correct the number of hours. My math is getting old with me.