If you have a coated car that surf will strip down all the wax from the car. Good if you want to prep the car from scratch for a new coating, bad if you are doing a maintenance wash.
Also, it is perfectly safe to have a clean and 'dry to touch' undercarriage with no diesel or any other motor oil applied to them. That is how they come from the factory and that is how you should keep them. In fact, the diesel would eat away the rubber bushes.