Hello. I'm new to the forum, and I'm new to knowing more about my car. Just started doing my own oil changes, and modifying different things for better peformance or protection. As I started to do research related to my own car (a Mazda6S) I started learning more about engine oil, transmission fluid, air filters, etc. One thing I've noticed is that there seem to be two camps of people concerning 5W-20 oil. Those that think a 20 weight oil is too thin to protect the engine, and those that don't. I don't want to start a big debate since I'm sure there have been many, but I would appreciate if someone could outline the basic arguments/evidence for and against 5W-20 oil for me. From what I've read it seems that modern synthetic or blended 5W-20s seems to protect just as well or better as any 5W-30. Why do so many people seem to think 5W-20 is ineffective if manufacturer's are recommending it? And for those that may ask, yes I did do a search of the forum. It didn't yield much.