Hi, all. You seem to be far more educated about oil than I am so I have a few questions. I have a 2003 Toyota Matrix (which is basically a Corolla) and it has 12,600 miles on it right now. At my first oil change I used some sort of Penzoil synthetic, I can't remember which. That was at 3,300 miles. Then, at 6,500 miles I used Mobil 1 15w50 because it was 90 degrees out and my wife and I were about to make a trip to Las Vegas and back from Los Angeles. I got almost 40mpg on that trip! Now that it's gotten a little chilly, though it's beginning to warm up, I changed it again at about 9k with Mobil 1 5w30. I've noticed that I've been getting horrible gas mileage. My wife will make a few short trips throughout the day, maybe 2 or 3 which are all less than 3 miles. We drive another 180 miles per week on the freeways. I've been averaging 24mpg within the last few weeks. I have a box of 6 gallons of Shell Rotella T 15w-50 because a friend recommended it to me for motorcycle use. Should I use the Rotella in my car? What do you all think? I'm looking for great wear protection in my car as well as improved fuel economy without spending $8 per quart.