ORIGINAL: TimBle
With ABC/N engines its always a good ideal to run a bit of castor (around10-20%, Morgans uses 30% in OMega) due to the lack of an oil ring to create sufficient hydrodynamic lubrication. In ringed engines, pure sythetic is fine.
Actually all engines, regardless of type, rely on hydrodynamic oil pressure because that's the only method to keep the oil film between the liner and the piston sealing area. In tapered bore (ABC type) engines the pressure generated expands the liner and compresses the piston so there's no pinch once the piston is moving fast enough that the oil doesn't have enough time to get squeezed out. Even at a very low idle speed the piston moves more than fast enough. Rings "oil ski"
over the film so the hydrodynamic oil pressure doesn't need to be as high as in an ABC because it only needs to be enough to overcome the ring tension.
Castor is better though because it likes to "stick" to metal so is much harder to squeeze out from between the working surfaces. This applies not only to the piston/liner but also both rod bushes and plain bearing crankshafts which all use hydrodynamic oil pressure to keep them seperated.