Synthetic oil is less susceptible to thermal breakdown and has more detergents in it than regular oil. My first oil change was to Mobil1 5w30, which is the correct weight for all Prii, and a Purolator Pure One oil filter. I had my selling dealer do that oil change, but I provided the materials. My cost out the door was a little of $11 including disposal fees and taxes, plus what I paid for the materials. All in all paying only for labor and having parts and fluids used that you prefer is a very good deal.

By the book the sump is 3.9 quarts, however I fill with 4 quarts. If you read this forum and a number of others you will find a continuing debate over fill amount. To put it in perspective, a 10th of a quart is about 2 shot glasses of fluid. So the difference between 3.9 quarts and 4 quarts is insignificant. The difference between 3.9 quarts and 4.5 quarts is significant. See what I mean. Also, your vehicle comes from the factory “overfilled”. Or rather, it is filled at the factory to 4 quarts. A slight “overfill” on your vehicle is not dangerous, not damaging and not risky. The factory wouldn’t be doing that if it were. I’ve heard people calling that a “break-in” fill and that it was meant as one time only, however there is no automotive history I can reference which supports that practice, especially with regard to Toyota’s manufacturing processes and how tight and exacting they are. In no circumstances would I under fill a crankcase. That is just a horrible thing to do. Oil provides cooling to the engine in addition to coolant. Lots of manufacturers have moved over to larger sumps on their engines so they can gain from the benefits of oil cooling. The only way you can have cooler oil running through an engine is if you have more of it there in the pan (or running through a dedicated cooler) than you realistically need.

Toyota’s oil change interval is 5,000 miles. That should be safe enough, but I moved to synthetic for that type of schedule. I’ve seen dino oil on cars that didn’t run nearly as efficiently as the Prius at 5,000 and I just don’t trust that schedule on conventional oil. The oil is too thick and too dirty. Synthetic seems to “hold up” better over the long haul.

For what it is worth, I expect many to come back and accuse me of being ignorant, or participating in back yard mechanicing regarding my recommendation to use the standard 4 quarts of oil in your change. However, it came from the factory with 4 quarts, my dealer put in 4 quarts, and the amount of oil above the full dot on the dipstick is so insignificant that I can’t see that having to dink around with the last quart of oil in trying to get a full to the dot makes sense.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)