oil change

What is your question?

Dear sir, Last time my serviceman has changed the engine oil i do not know which oil he has used whether regular or synthetic. Now i have couple of tins of synthetic engine oil can i use it W/O knowing about previous one or i should flush out previous one before using synthetic one.Secondely it is necessory to use manufecturer's recommended brand only? Pl guide me in this regard and oblige. Thanking you, Nilesh Mehta.