The simple answer is for your type of driving, is to change your oil every 6 months like the owners manual says for severe driving. If you go to a quality place, every 6 months you will get all fluid levels checked, air filter, tire pressure also. If you want to go longer oil drain intervals. at 6 months take an oil sample and send it off to a quality oil analysis lab to see what condition your oil is in, and then base your oil change intervals off of what they recomend. Every body drives differantly, under differant conditions, and every engine behaves differantly. What oil drain interval that works for one car may or may not work for another. If you want to extend oil drain intervals, the smart thing to do is take a sample and have it tested. You may need to to this a few times to get a good idea of what is a good oil change interval is for your car under your particlular driving conditions.