Peter
I use only 20% castor in all my engines. Castor is the best preservative against rust that you can get but after a few years it can start to thicken and go gummy. The varnish that you see sometimes is a lubricant in itself and is the final protection against a very lean run if the temp goes so high that even the castor starts to burn.

The only thing you need do for protection (given that you use castor) is this. On the last flight every day run the engine at full throttle then take off (preferably) or pinch the fuel line and wait for the engine to run out of fuel. If you take off the fuel line be aware that it'll spray fuel out because of muffler pressure if you don't pinch it shut. Wait for the pressure in the tank to bleed back into the muffler before reconnecting the line.