Hello everybody sorry I have not had time to contribute anything to this forum for a while... Question: I was keen to try a Castor/Synthetic oil blend in my well worn in 70cc engine to extend its life so I purchased the first Castor based oil I could find which is Castrol r30. At 50% mixed with my regular synthetic Motul 800 oil the r30 seemed to lean up the engine dramatically raising the idle speed, also the fuel consumption reduced dramatically! interestingly instead of causing the engine to overheat is actually kept the engine much cooler than before. Plus there was a major boost in power. I ran it for a little while like this before I discovered that the jet and pin in my carby was gummed up hence the lean running. After a cleaned carby and at 20% the castor seems fine and the engine still seems to have a better ring seal than before BUT. Massive quick build up of a strange brown carbon deposit on the piston top. This won't clean off with acetone!! I have chipped it all off and had a look at the rings which seem clean but it certainly is a pain to remove this all the time. After some net research I have seen many mentions not to mix Castrol r30 with other oils... Though not much mention why. Then I read many recomendations here on this forum that mixes are fine. Is it just my type of Castor that shouldnt be mixed? Is the funky build up a result of mixing oils or would it be the same (or worse) with straight Castor? Would a pre-blended Castor be safer and is this little experiment just not worth the risk?