I have three Mercedes, 90 350 SDL (for sale), 85 300D (will sell soon), and a 85 300 TD, are there any diesel fuel additives that are good to use? I ask because the cars have very low miles for their years, so they have sat a lot in their lifetimes, so if the biodiesel I am running doesn't get the fuel in shape, what commercial additives are available that are good? Thanks