Keep the castor-based lubricant. You are relying on lubricant film-strength & it is very much more important at low temps. Castor has noticeably greater film strength than syn at low temps. If the oil was being circulated, as it is in a car, then the syn would be the choice because it will flow more easily & circulate quickly, but in a 2 stroke you are relying on lube in solution. The weaker film strength of the syn lube puts the 2-stroke engine at greater risk in cold weather.