Bhai, no need to flush the engine before using a different type of oil provided they r of same viscosity. U can switch back from mineral to synthetic as and when u want. They r compatible with each other. What do u think a semi synthetic oil is? : A blend of synthetic + mineral.
To OP, I have just switched to 5w-30 in my city steermatic 2007 with 78K on odo and it has started to feel very light. The oil was Honda SM 5w-30 fully synthetic made for Honda by Total. Retail price PKR 2400/- I was told that its good up to 10000 KM but I plan to change it on at-most 8000 KM.