I just completed my first trip into work (42 miles) on the beltway with my new Sonata GLS Auto 2.4. Sweeeeeeeeet.
I read the owners manual and it suggested that for the first 1200 miles I keep the speed at or below 55 mph, avoid sudden stops and starts and vary the speed from time to time. I will attempt to do this for 1200 miles. Tell me if this is good to do for the car or even necessary. Also, would it benefit the car to use synthetic oil? To get a K&N cold filter?

Man, it rode like a dream. The trip computer says I averaged 30 mpg (primarily why I got the car) and it was quiet and smooth. All you jaded and seasoned owners please don't harsh my mellow about this car at least until I've gotten the first scratch on it...

 

save your money use regular oil.

 

Just drive normally and avoid full throttle and your car will break in fine. If you change your oil every 3,000 miles regular oil will be fine. I put 134,000 miles on my 99 Accent and 95,000 miles on my 04 Elantra. I put 5W-30 or 10W-30 Walmart brand synthetic oil in them and had no engine problems with either. The Walmart brand synthetic oil in the 5 quart bottles are a great value. I found that I could change the oil myself with synthetic for about the same price as if I payed someone to change the oil with regular oil. The only real advantage of synthetic oil is in very cold or very hot weather. Synthetic oil doesn't thicken up in cold weather and is easier on the engine durring startup. Synthetic doesn't break down as easy as regular oil in hot weather. I changed my oil every 5000 to 6000 miles with the synthetic. I have no experience with the K&N air filters. I just bought my 2008 Sonata on Dec 23. Mine is the GLS with the 4 cylinder and 5 speed manual trans. Yes the Sonata is really nice compared to my 04 Elantra and gets almost the same fuel economy. So far I am averaging 32 MPG in my 100 mile a day commute to work.

 

Opinions differ on how to properly break in an engine. The advice generally found in owner's manuals is the same as it was decades ago. Some people argue that with today's engines, owing to their tighter tolerances, the best way to break in an engine is by running it hard, with smooth transitions but at full throttle.

There's little if any recent research on the issue that I'm aware of.