Like a lot of aspects of this hobby, you'll find people on both sides of this one that are very, very adamant about their particular stance. I'm pretty adamant about mine, too. My opinion, based on what I have experienced, is that it doesn't matter. Straight castor, a castor-synthetic blend, or just synthetic--they'll all do the job. Ken is spot on when he says that none of them will do the job in case of a lean run.

As for the percentage of nitro, I'd wager that most of us run 10-15% in our more common engines. In other parts of the world that's a bit different, with lower (or no) nitro being used. That mostly has to do with expense, and has led to slightly different compression ratios being used. Here in the States, having someone suggest that 15% nitro is a dangerously high percentage is somewhat laughable.