Not true--
I use all synthetic fuel in my helis with no after-run.
No rust, no corrosion, build-up of gunk or bad bearings.
New synthetics have additives that coat and protect metal surfaces without the gunk and build-up that Caster leaves.

All my engines that are run on caster or blends have these problems.
They must be drenched with after-run over winter or the bearings will be rusty by spring.
I have too many planes to store in the house so they hang in the garage all winter, which doesn't help.
I used to swear by Caster because I'm from the old days when it was the best you could get for a glow engine.
Synthetics are proving to be a better lubrication in all forms of racing these days, whether engine or driveline.
Our glow engine are no different.

These are two engines with similar run-time and age---see for yourself.
Kirk