Facts: At about 100,000 miles, I went from no oil usage to about 1 1/2 quarts / 5000 miles Have changed oil every 5k miles Used bargain oil More then the oil usage, it intrigues me as to what the cause could be. I spoke with 3 different Toyota mechanics and here is where I am at: Eliminated the PCV valve as cause. I was told that they have never replaced one and that if it was stuck, I would have a fast idle which would lead to a "check engine" light. Don't think the sudden change in oil usage could be due to wear (rings,cylinders) - too sudden. An interesting possibility suggested by one of the mechanics (who certainly appeared to be one of the more educated of the 3) is that the oil rings are stuck (he claims due to the use of bargain oil). He says he has seen this often on Toyotas (if not the Prius). I like this explanation because it addresses the sudden use of oil. He says that a compression test is useless because it only measures the compression side (rings). Any thoughts? If he is right, what do you think about adding 1/2 quart of something like Marvel's Mystery Oil a few hundred miles before changing the oil to see if they could free?