There are always two manufacturer recommended maintenance intervals that are located in the vehicle's owners manual. The regular maintenance interval is intended for climates without extreme temperatures or when the vehicle is not used on a daily basis or for sever duty, which is typically every 5,000 to 7,500 miles (depending on make and model). If the car is driven on a daily basis or in an extreme climate, adhere to the severe duty maintenance schedule which will usually be every 3,000 or 5,000 miles. With conventional oil the service interval should be every 3,000 miles or 3 months. With Synthetic oil a 5,000 mile interval is permitted. Most places in the US would be considered Severe Duty.