So I am thinking about buying this 1996 Nissan 200SX SE-R, but I am a little confused about what this guy is saying about it. I have looked up the Nissan specs and with the stock motor its supposed to get around 31-33 mpg. This guy is saying "The motor he replaced it with is a newer generation SR20DE which is more efficient then the stock engine which means better mpg and more performance" I expect the better performance, but my friend read somewhere that the SR20DR actually decreases the mpg. Though supposedly he was able to get 40 mpg on a long trip up the Oregon coast. Is he telling the truth or is that much of the story BS? What kind of mileage should I expect out of this car?

The mileage shouldn't change if he replaced it with another SR20DE motor. It may increase by 1mpg or so, because it may run more efficiently because it is a new motor with less mileage. Nothing else special about it though.

New version out of what car? If it's the roller-rocker SR, he might see a little better mileage, but it's probably mostly due to having fewer miles and probably in a better state of tune.