Burning question I have.... I am not 100% sure that thinner is better, nor do I think thicker is better. I am in the middle. My question is this. My Mazda3 is NA, and specs either 5w30 or 5w20. The turbo version of the car specs 5w30, which makes sense due to higher temps, etc. Many use Rotella 5w40 in the car with good results. So my question is: Besides the turbo, the cars are very similar. So, what prevents one from using a heavy 30/ thin 40 in the NA version of the car? Any terrible consequences? Just curious, as I am looking to keep one oil in my garage to fit all sizes. Thanks.