i had used castrol magnetic 10w40 for about 2 years, its good oil give me good 2 years of running but some how from past 6 months, i felt after 3k engine starts giving extra noise which significantly reduced when i change oil ! some how i felt whether the quality is down or some other factors, btw car is 2006 city. So i switched to zic A 10W40 and felt the difference, car sound remains same till 5000 km., btw, i always change castrol and currently zic at 5k km, not more then that , sometimes if i am outa city then stretch it to 6k but not more then that but only twice i remember from past 3 years.
my suggestion, keep the air filter clean .... always, clean it even after 500 km if you feel u traveled in some dusty environment ! keep the breathing of car clean it will help with good mileage + keeps engine oil in good health too !!!!!