Ok on the fuel one, he is telling you the truth, you get a mold forming between the water that collects by the drain points and the fuel, this is called hormoconis resinae if memory serves me right and produces a mold similar to the stuff you find on window frames... this is seriously bad as it can block filters and seriously damage tanks etc.....Therefore frequent draining of the water from the tanks helps prevent this.. the little blighters live on the water and feed off the fuel. Being in the climate you are in ( Malasia) it will be even worse, so there dilligence is beyond reproach..
please read for a more accurate description
http://www.conidia.com/downloads/INF...0SHEET%201.pdf
http://www.cabi-bioscience.org/docs/...bioscience.pdf
As for streaking I can totally agree with them, especially if it is warm.....
In the shade and cooler away from direct sunlight it will be easier to clean and prevent streaking forming due to it drying rapidly..
I would go with they are telling you on both counts.