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.
One real concern is a very high mileage car that has used conventional oil going to synthetic. In that case,the synthetic oil might dissolve too much gunk, so it is recommended that you make the transition with 2 oil changes at 2K miles each so as to flush out the gunk.