I don't know about having a minimum of 1/4 tank fuel, I just ride and drive

I've been in several situations when I fill-up for fuel upon seeing the orange light low fuel indicator go off, and I never had problems.
As to the filth in the tank, that's the job of the fuel filter. Also, from what I know, the fuel tank doesn't have anything to do with the CRDI system as it is just a reservoir for fuel. I guess what they're trying to say is to avoid really running out of fuel (which can also happen when the vehicle is tilted, like in 4-wheeling) because the ECU for the CRDI system might go awry. And the only way to prevent this is having the habit of maintaining a minimum 1/4 fuel tank level.
Just my 2 cents.....