Originally Posted by ChrisSlicks

The service manual calls for 3750/3 for heavy use, 7500/6 otherwise. Note that most of us fall into the first category and hence should follow the 3750 or even less if it is being tracked.

They describe the heavy use scenario as follows:

Follow Periodic Maintenance Schedule 1 (3750) if the 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.