I read some comments about the Prius that the ICE rotates at a low rpm when cruising or coasting even when the gas is shutdown, I guess to make starts easier if they become necessary. If thats really the case then I guess it would make sense to have a very thin oil, as you would need to lubricate an ICE that is not running all the time and therefore might not be operating as hot as a normal ICE. Just my guess...