The normal thing is as you say, change every 50,000 km and in the purchase of a use used to make the maintenance right away. But there is a missing detail, not only the straps are changed to mileage but also for time. The limit of a time distribution strap would be about 2 years, due to the issue of dryness, many are confident that they have 20,000 in two years but can break the same for the elapsed time