What is the traditional commercial driving rule of thumb regarding gear selection before starting down a steep mountain slope ('上坡用几挡，下坡用几挡')?

1. Select a downhill gear that is equal to or lower than the gear required to climb the same slope, engaging it BEFORE descending
2. Select the highest overdrive gear to allow the truck to coast freely with minimal engine friction
3. Always start descending in top gear and shift down only when the truck reaches 80 km/h
4. Keep the transmission in neutral and control speed exclusively with the parking handbrake