Jan,

Hear what you're saying, but I'm afraid I have had some bad encounters with synthetics - and many of my friends also!

Basically, when setting the main mixture needle, if you set it too lean with a castor oil fuel the engine will just stop. However, if you do the same with a synthetic fuel what happens is that the engine keeps running - so the modeller thinks all is well - and flies - only to find that during the flight as the fuel gets lower and the mixture leans out, the engine suddenly starts sounding a little 'strange'.

On landing, you find that the plating off the piston/cyclinder has been damaged due to the synthetic allowing the engine to continue running in an increasingly lean state.

...God doesn't make mistakes - I'm happy to stick to nature's castor as a lubricant.