I always error on the side of safety... It was hard enough for me to go 5000 miles. The way the big fleets do it, is they run, constantly monitoring fuel economy, then when it drops, they dump the oil. I had a Crown Vic. I tried that concept, and at 2700 miles, the fuel economy dropped (conventional oil) and at 3600 it dropped with synthetic. Now the mileage wasnt exact numbers, so, but it was in that range.