The piston/cylinder fits of most Surestarts is poor compared to previous Cox efforts. There IS a lot of blowby, in fact about one in ten Surestarts are good, the rest are fairly poor. I believe this is why Cox specifies 20-25% nitro these days. The production cylinder previous to the Surestart seemed to be better, but the cylinders/pistons made before the slit exhaust era are good to excellent.

Which leads me to another point, taper. The amount of pre-slit engines I have that exhibit taper is very surprising considering tapered bores were only claimed for the TeeDee P/L. I'm talking all types, from number 1 cylinders, numbers 2's , etc. exhibiting tapered bores. Anyone else noticed this?
Not one slit exhaust engine that I own regardless of configuration shows any sign of it.

Oh, to remain on topic, another good reason to avoid 100% synthetic oils is crankcase bush wear. Castor oil seems to help there.