mobil 1 15w-50,i changed the oil at 2500km and it still feels new(the oil after pouring out),did an oil change at 5000km,and oil still feels the same....engine has done 40000km,still feels good,i will see the wear when i change to the 65mm head and block(engine has never been touched).synthetic oil are meant to last a long while so i don't understand why change so often,maybe because the engine feels rough but bear in mind that the fuel quality some times have this effect,this always happens to me so i try and avoid centain brand of petrol station.i think fully synthetic oil can easily last 10000km on our engine but that just my opinion....