when should I change oil

I put in castrol syntec in my cav. I was told I don't have to change the oil every 3000 since I use that oil. Is this true and if so when should I change the filter? Thanks

It is not 'necessary' but is usually a good idea to change it anyways. However with that said if you wish to wait longer to change your oil, it is still recommended to change your oil filter at regular intervals of 3000 miles. Synthetic oil does not decrease the amount of contaminents trapped in the filter, they still only hold so much.