The Toyota Prius 2007 Owner Manual states on page 432:
ENGINE LUBRICATION
Oil capacity (drain and refill), L (qt., lmp.qt.):
- With filter 3.7 (3.9, 3.3)
- Without filter 3.4 (3.6, 3.0
However, it states on page 404:
The approximate quantity of oil needed to raise the level between low and full on the dipstick is indicated as follows:
1.5 L (1.6 qt., 1.3 Imp.qt.)
Why is the oil capacity more than twice larger what's needed to raise the level between low and full indicator on the oil dipstick?
If the volume of oil contained in the engine was the same as the volume of oil between the low and full indicators, it would mean that there would be no oil remaining if the level dropped to the low mark.
By having more oil than that indicated between marks, there's sufficient oil to continue safe operation of the engine and to allow continued operation to travel to the location necessary to add oil to the full mark.
I have seen oil indications, or rather, I have seen no oil indications on a dipstick, well below the low mark, yet there was still oil in the engine and no harm was done.
Oil provides for cooling as well as lubrication. That's especially valid in systems known as dry sump lubrication, where the volume of oil in the very shallow oil pan is only the amount required to provide appropriate splash lubrication, while the tank contains a substantially greater amount.
Yet again, more oil provides for a greater level of dilution of contaminants such as particulates missed by the filter (or not yet filtered) and moisture from condensation or other sources.
Motor vehicle engines have evolved over the years and engineers desire to keep costs to a minimum (less oil means less expense) but also to keep the lifespan of the engine as long as possible.
If you take your temperature with a glass Celsius fever thermometer, normal temperature is 37C and the lowest mark is about 35C. The scale doesn't go to zero because if your temperature is below 35C, you are already dead, or you will be soon.
By the same token, there is no good reason to have a scale on a dipstick that goes down to zero oil, even though it is possible to make a longer dipstick that touches the bottom of the oil pan.
With normal care and maintenance, the oil level is kept between the two lines, representing FULL and 1 LITER LOW, which may be 75% to 80% full. We don't really care about seriously low oil levels because we never allow that to happen.
