18,000 mile's on a oil change is a long time and could very well do in your timing chain. There different service interval's for different driving condition's. Like severe you would change your oil every 3750 mile's. Here is severe driving defined see if you fall into that interval.
Follow the Severe Service Schedule if your driving habits frequently include one or more of the following driving conditions:
* Repeated short trips of less than 5 miles (8 km).
* Repeated short trips of less than 10 miles (16 km) with outside temperatures remaining below freezing.
* Operating in hot weather in stop-and-go "rush hour" traffic.
* Extensive idling and/or low speed driving for long distances, such as police, taxi or door-to-door delivery use.
* Driving in dusty conditions.
* Driving on rough, muddy, or salt spread roads.
* Towing a trailer, using a camper or a car-top carrier.
Tuesday, December 21st, 2010 AT 6:51 PM