It is, in a sense, oil - traffic/oil film, smoke emission from vehicle, etc. Something like how your skin would feel and look when wiping with a tissue paper after a few hours walking around in the middle of KL. You should be able to easily remove it with a light polish or an All-In-One (AIO) like Duragloss 501, 3M One Step Cleaner Wax, Meguairs Ultimate Polish or the like. You can also strip it with a 'chemical wash' using a mild all purpose cleaner, but just remember to wax your car again afterwards.
I guess a white car also. See above.
hhrrmmm.....define reasonable please... ?

Aisay...easy way out (but also with higher risks) is using an all purpose cleaner....spray and wipe with damp cloth, then wash normally afterwards to remove/rinse off the apc. Cloth would get dirty very quickly...clean/rinse the cloth regularly (dirty cloths creates swirls...especially when you're removing these kinda dirt, but I don't think you would mind much about it). Avoid working under the sun...and don't expect the paint to be shiny afterwards....but it would look very clean.