Before this big, bad pickup arrived, manufacturers largely used upgraded versions of their light-duty trucks to handle heavy-duty work. The 1999 Super Duty was a separate model line with its own style and hardware underneath.
You can run a conventional oil 5K miles but you should be able to run a synthetic oil 7500 to 10000 miles. Royal Purple API-licensed synthetic engine oils allow motorists to travel as many as 12,000 miles as the recommended oil change interval.
I guess I just know too much based on personal experience. Here are my revised oil change recommendations: Change regular petroleum-based oils every 4000 to 5000 miles and synthetic oils every 5000 to 7000 miles.