Well if you are a car owner in Karachi, then you are at a disadvantage because Tap-Water is not available in abundance like in Lahore, Pindi, Faisalabad etc. Hence Boaring/Tubewell water is used in most of the Service Stations in the City.
The nature of this sub-soil water in Karachi is such that it induces rusting of metal and erosion of rubber and plastic at a much faster rate than normal tap water.
To counter this the service station people here use diesel or oil. Oil provides better coating than diesel. But being more viscous (sticky and thick) than diesel it catches dust even more rapidly. This dust creates a layer that eventually erodes your cars underbelly.
Diesel is a better compromise, in this regard.
Ideally the best way would be to wash the car engine compartment and the underbelly. Then have it completely dried up by cleaning and/or by using air jet ... before taking it out on the road. Use oil and grease for lubrication your shocks and suspensions.
However this is easier said than done. No service station team would go through the rigours of completely drying a car on its underside. Too much time and effort required. So it is a dilemma and we have to live with it.