I have Castrol 5/30 regular oil in there now . Do I have to run any engine flush through the system to remove the residual Castrol oil before putting in the synthetic ,I dont know if i just drain the old oil and put in the synthetic that they might clash with each other, if there is any regular oil left in the engine or is it something not worry about
thank you
By the way the engine has no leaks and the underneath of the oil cap on the valve cover is spotless with no sludge.
2003 accent

 

There is no need to flush the engine. Just drain and fill like a normal oil change. Mixing synthetic and "regular" oil is not a problem.