I looked at Greasel's system and another one (greasecar.com , I think), but didn't like the fact of the extra space you give up. Plus, you have to start with diesel, switch to oil, then back again before you shut off your engine. It was running about $700 dollars plus cost of installation.
If we didn't live in the city, I would make biodiesel. The equipment is not that expensive and you don't have to modify your truck.
There's a number of threads on the TDR where guys talk about filtering waste oil (vegetable or motor) and then mixing a percentage of the filtered oil with their diesel. Some just let their old motor oil sit, settle, and use it. If you're using a good oil filter, like a Stratopore (sp?0 that might be OK, but I think I would filter anything I before dumping into the tank.
This is the way I'm leaning right now. Start doing my own oil changes (save money there), filter it, store it in jugs, and put some in with each fillup. You can adjust the amount of oil added based on current local temps. Also, you could carry jugs of the filtered, used oil on trips, if you wanted. Just add a jug or two when you stop to refuel.
Hope this helps,
Jim