AFFF is the stuff to use, its a chemical that is added to water and very effective on petrol fires. Did some fire training with AFFF and couldn't light a pool of petrol that had a layer of AFFF on the surfice even when lit matches were thrown in
MCA recommend 2 x 1 kg and 1 x 4kg plus 2 buckets for a small workboat.
Your right dry powder makes a real mess when its used. Although you have petrol the thought of dry powder going into a turbo diesel doesn't bear thinking about.