Strangely, this is one area where the Prius is not as green as it could be, IMHO.
Toyota recommends changing the oil every 5k miles. Yet, as computerized as the Prius is, it would be so easy to monitor the number of cold starts, and a few additional parameters, and have the computer indicated oil changes as required. If you combine that with synthetic oil, you can easily stretch change intervals to around 15k miles.
My MINI Cooper S had that, 1st change was at 12k miles, and subsequent ones at about 17k mile intervals. All BMWs do that, and they do build the best conventional ICEs in the world, afterall.
Reducing oil changes would make the HSD just anotehr tad greener.