If the wrong type of oil was put in the toyota prius, well that harm the car?
It will if toyota get to know that you have put additive in the oil .
No. The warranty is required by law and is never voided no matter what you do. But they can argue against any particular repair you may want them to do, if there is a problem later. So it is better not to do anything they will be able to argue with, or at least do not tell them about it. There is no reason to put an additive in the oil, or the wrong kind of oil. The recommended oil is likely the best, and additives can harm seals.
An additive won't void the warranty, but it won't do any good either. Not one aftermarket additive has been shown to help regardless of any anecdotes, celebrity endorsements, or late night TV shows you may have seen or heard. Instead buy the best oil you can. You can tell the best oils because they meet both ACEA A5/B5 and ILSAC GF5 standards. It will say so on the bottle. Putting in the wrong oil can harm the engine although it depends on the circumstances. If the oil is too thin, then it may not lubricate properly at higher temperatures. If too thick it may not lubricate properly at low temperatures. Your 2008 Prius uses 5W-30 which is commonly available so there should never be any reason to put in the wrong oil. If you overfill the oil (oil is over the overfill mark on the dipstick) you can harm the O2 sensors. Some people say that you get less mpg. The best plan is to have the oil level at about 1/4 inch below the overfill mark.
