Conversation Between kin quick and datuck

hi i put semi synthetic oil in my subaru impreza wrx 2003 after buying it. the oil was quite low but still enuff on the dip stick. i notice on the service sheets the car always had 5w 40 synthetic used always. since putting the oil in i seem to be suffering from a knocking/tapping noise at low revs when cold. i think it must be piston slap as it dies down when the engine is warm. but seems strange how its started since ive put the new oil in. is it the thinner oil or part synth that has caused it? what can or should i do to stop it.