I use Manahattan Gmat materials, and have gone through all the basic sections of the Numbers Properties, FDP, and Equations guides, including the end-of-chapter problems. I want to do Sackmens fundamentals and then hit OG part 1 questions, then move to the Advanced section and finish OG part 2. After that, I will try the challenge sets and extreme challenge sets. (That's gonna be a war!) I find I am not that off from alot of answers I get wrong, all it takes is one tiny error, I just need to sharpen up.
This might sound a tad stupid, but when you refer to OG "difficulties" - is this a feature Sackmen offers? I mean, if its the OG - what do you mean by these sections? I just have one book, with one area called "Problem Solving" and one with "Data Suffiency", no specific level arrangement