Use the cheap stuff, and thicker single weight. The synthetics are good for resisting wear and breakdown of the long carbon chains that give oil it's protective and lubrication qualities. Sitting over winter has none of the characteristics for which synths might be an advantage. A thick single weight will cling better than a multi-weight and thus stay on the metal you wish to protect instead of running off and sitting in the sump where it does no good. In the spring, pour a tablespoon of oil in each cylinder (through the spark plug holes), turn the engine over a few times without starting then come back in an hour and start your bike. Let it warm at idle for ten minutes, take a short ride to really warm the oil, then change the oil to your standard choice.