Realizing this isn't the USA, I thought I'd post this anyway so people can see what happens out there. Who knows what filtering is done in gas stations in other parts of the world or what happens when you transfer it between gallon jugs or dirty funnels.
I drew 25 ml of diesel fuel that I bought at the gas station next door through a little patch of 4 micron filter paper and took this picture with the macro on my camera (to the naked eye it looked clean)
Then I put it under a 10X microscope to look at a little portion of it (you can compare the green squares to get an idea of the area)
A lot more for those that understand Spanish on this link Diesel filtration