i dont think anyone makes 15w-40 full syn. you can find 5w-40 full syn, which flows better when it's cold. In theory and lab testing, synethic oil is better than conventional. It maintains its grade longer, at higher temp, it evaporates less, and flows better. However, in the real world unless your car is designed for syn oil, it doesn't mean that your car encounters extreme conditions where it can take advantage of these benefits. thus, no one can truly make the claim that synthetic is "better" for your car.
overall, 15w-40 is a good grade, and you have lots of options, such as Shell Rotella, Chevron Delo, Mobil Delvac....etc
Last edited by mnguyen84; 11-23-2012 at 08:30 AM.