Aluminum tanks will last forever, but the most important difference, especially in a climate like Denver's, is that you're going to want to heat your WVO in the tank because it'll get really sluggish and your fuel pump will likely suffer if you don't. And putting a heating element in a plastic fuel tank is dicey at best. If you can figure out a way to heat the oil with hot engine coolant in your plastic tank, then maybe it's not such a big deal, but I haven't seena a convincing way of doing that yet.