Green Earth Technologies has developed a motor oil made from plant oils and beef tallow. Not only is the oil made from renewable feedstocks but the company claims the oil is biodegradable. By adding a product they call G-Disposoil to the used G-oil, it is broken down and can apparently be just poured on the ground to biodegrade. G-oil was just recently launched and it's not known where you can get it yet or how much it costs, but it's worth watching.
[Source: Green Earth Technologies, via Gizmag.com]