The oil companies used to make an engine flushing oil,it works well,see if it is still made.Also there are many oil additives etc/engine cleaning etc on the market try these also.I have had a policy for over 30 years of changing the oil every 3000km,it works,I have not had an engine problem ever.When you think the engine is clean change the oil every 1000 km once or twice,with a bit of luck all will be well.I was told long ago(dont know if its true!)that oil cannot be broken down,only contaminated.When I took the rocker cover of a Holden Gemini,(my daughter totalled)150000km the insides were a golden colour as new.David
