Well this one always starts an argument but in my opinion castor is by far the best. When a synthetic gets too hot it vapourises and burns. When castor gets too hot (at a much higher temp than synthetics) it has the happy knack of turning into another lubricant. Castor also "sticks" to metal better so it doesn't tend to get squeezed out of highly loaded areas like the conrod bushings. The "stickiness" is also why it stays on things like bearings and protects them from rust.