While I'll agree that with care you can get away with using an all synthetic for running in, there are two areas of the engine that will definitely benefit from having a little castor in the mix and that's both ends of the conrod (which also need to be run in). Castor polarises to metal which, although not strictly accurate, can be thought of as being magnetically attracted to metal. That means it wants to stay in contact with it. Synthetics don't work this way which means it's much easier to squeeze it out of running surfaces. There are some synthetics which supposedly have a polarising additive but it's probably just a bit of castor anyway