With proper care, synthetic lubricant is not bad for your engine(s).

Most lubricants do funny things under heat.

Most synthetics, including those used in model aviation, burn or flash when a specific temperature is reached. A lean run could cause all the benefits of synthetic lubricant to go away, leaving your engine with no lubrication.

Castor simply becomes a better lubricant as it gets hotter. Castor does leave a little caking.

Synthetic/Castor blends are designed to give you the excellent lubricating qualities of synthetics and the thermal protection of castor, without all the mess. You will still find an oil film on your plane,[&o] but you will not have to squeegee it off.

Model engines are prone to do things without warning or logical cause, including running hot on just one run. A little castor is cheap insurance.