During the first few hundred miles, it's best practice for any engine to vary the rpm and load. This helps mate all the wear surfaces together properly: piston rings and cylinders, bearings and cam/crank journals, etc. This isn't just a Prius thing, it applies to any vehicle with an internal combustion engine. Running an engine constantly at the same rpm for many miles during the break in period is not fantastic for it. That being said, a modern Toyota engine is not the same as a domestic engine from 30 years ago when this break in procedure was critical. Back then, you could ruin an engine if you didn't break it in correctly. You're not going to be able to do that today. Even if you do everything wrong, the worst you're going to do is reduce the lifespan of the engine by a little bit. Instead of being able to go 400k miles before needing to be rebuilt, it might only go 350k. Changing your oil every 6m/5k is going to have a far greater effect on the lifespan of your engine than the break in period. That could be the difference between 350k and 125k. But if you want your new Prius to have the absolute best chance to get every mile possible, vary the speeds during your trip home. If you can, avoid interstate freeways for some/all of the route and use two-lane roads that force you to vary speeds. And feel free to accelerate to pass people; that's actually beneficial because it changes the load the engine is under. Don't be focused on getting max mpg on this first trip; that can come later. The other thing you can do if you want to maximize the life of the engine is do your first oil change after you get home. It won't dramatically change the lifespan of the engine, but it will help it just a little bit. In the end, it all depends on how long you plan to own the car and your personality type. If you only plan to own it 3-5 years, then none of this matters to you personally. Any negative effects will occur long after you've sold the car and moved on. In that case, the only reason to do this stuff is because you like to do things the best way they can be done. That's meaningless to some, hugely important to others, and everything in between to everybody else. You do you and don't let us tell you otherwise.
It says do not drive at the same speed for long periods in the owner’s manual: ■Breaking in your new Toyota To extend the life of the vehicle, observing the following precautions is recommended: ●For the first 200 miles (300 km): Avoid sudden stops. ●For the first 600 miles (1000 km): • Do not drive at extremely high speeds. • Avoid sudden acceleration. • Do not drive continuously in low gears. • Do not drive at a constant speed for extended periods.
It can be an indication for how much dirt is in the oil. For lubrication purposes, this isn't much of a 'thing' because as it turns out modern engines have oil filters to take out the big hunks. HOWEVER (comma!!!!!) Something is causing contamination in the EGR loops and in Variable valve timing, and other systems in a lot of not-very high mileage engines. This contamination occurs when dirt, debris, or sludge in the engine oil accumulates in an engines small oil passages and solenoids, which is why it can be beneficial to change the oil a little more often than the 7.5K to 10K oil change intervals that the OEMs are recommending. This isn't a "break-in oil" thing - which among car enthusiasts is the equivalent of "nursery water" for a first child. Most of the people who are worried about this probably trade out the car before it gets to 150,000 miles and even if you keep to the OEM maintenance practices and periodicities, and SKIP an oil change or two you're probably going to see 150,000 miles in a car without many problems.
