RE: Why synthetic oil?
Oil viscosity at temperature is easy to figure out. Look at the spec sheet for Klotz Benol and look at the spec sheet for their synthetic lubes. You will find that they are very similar at operating temperature of 100°C. Oils are not normally rated above 100°C and that's where knowing the viscosity index lets you figure out what it will be at 150°C. But we have to realize that viscosity is not the only thing influencing lubrication. Ester based lubricants are "clingy" and adhere to metal very well.