For well known trails in areas like Moab, Colorado, etc, its hard to beat the Charles Wells series of books. He gives the trails difficulty ratings and there are excellent descriptions so you know what to expect. These are well worth the money.
This condescending dipshit is a tool. I don't care that he doesn't agree but for someone supposedly so smart, he's remarkedly close minded about the whole subject.
What is gained by going against manual recommendations? Anything of significance? 50 extra horsepower? 100k more miles of life? 10 more mpg? No, and in the end who really cares that much about engine oil viscosities? Run what the manual says and call it good.
Now, if someone really wants to be...