i've seen that silvery paste in high mileage gasoline gas tanks before; i came to the conclusion (might not be correct) that the paste is from the gas pump nozzles; every time you put the nozzle in the filler hole there are some flakes (silver metal) coming off the nozzle and over time they accumulate in the bottom of the tank. since they're always suspended in gasoline (or diesel) it kinda makes that silver paste.