I used to run 10W-30 in the summer and 10W-40 in the winter for my cars. It was what the manual and dealer recommended and help my cars run to over 200,000 miles. My current car requires 5W-20 year round. Once I had someone else change my oil because it was actually cheaper then doing it myself. They put 10W-30 in, and the engine ran really bad, and the fuel milage crashed. I started looking into what could be the problem by what had changed recently. Found out what the mechanics had done, took it back, explained the situation, and they did another oil change no charge putting the correct oil in this time. Car ran smooth as silk after that. I haven't let anyone else touch my car for basic maintenance since.
Lesson to be learned: If you use the right oil, the engine will last forever.