I have tried all sorts of mixes for sport flying (mostly 46 2S) and eventually settled on 5% Castor, 10% Synthetic for my oil content.
This was not based on is synthetic better/worse etc, but based on COST.
castor costs me $10/litre and synthetic costs me nearly $17/litre.
For me, castor is cheaper than synthetic to but, but I hate the mess, so I played around with the % and came up with something that was as cost effective as possible and not to messy.
Once an engine is broken in, I always start reducing oil % from 20% down. I have never had a problem with 15%, but I am always careful and try and not run lean.
I don't know about anywhere else, but in AUS, If you fly more than once a month, and you don't mix your own you are virtually burning your cash!