The clean engine shown has had quite a few lean runs and some very bad abuse when it ran wide open with no load after a crash.
It had a near meltdown when the case line for the Perry pump broke causing a very lean condition. By the time I got it down, it melted the plug and was EXTREMELY hot. No burnt piston, scuffing or dis-coloring. It's had quite a few cases of fuel through it, is clean as a pin, all original, and runs like new. It has never had after-run or run dry and is stored right next to the planes. Notice that even the exhaust port and the inside of the crank are spotless. I've NEVER seen that with Caster! My other two heli engines look just like this.
All the guys I fly with, from beginner to expert use 100% synthetic in there ABC and ringed helis. I've seen plenty of lean runs and seldom any damage from it.
In my experience, the engines are protected just as well as with Caster.

The other engine just had the bearings replaced a little over a year ago and are already rusty. It's not run on all Caster but a blend. Though most aren't this bad, all my engines' run on Caster look like this even with some after-run in them. If the Caster gum is suppose to protect the metal surfaces, why are they rusted and pitted when they're cleaned up, not shiny and smooth? Sure, Caster has proved to protect against lean runs, but a lean run has never caused me the problems that rusty bearings have over the years.

I've heard a million different theories over the years as to why Caster is good for an engine. Up until recently, I was an advocate too. I just haven't seen engines this clean and shiny with this much age, use/abuse and the only thing that is different from the rest is the oil. It's just what I've experienced.

Kirk