Rumor has it new NBO2's are more accurate at stoich than new WBO2's, and also that the NBO2's should be trusted for everything except PE. For PE, you need to worry about making sure your HPT AFR readings match up across the entire range with what your WB scanner is reading. I consider the latter a real PITA. Even if you use WBO2's to tune in OL (which I've done also), you still need to insure the readings are accurate before you start... and supposedly you need to re-check regularly.