Most people use the medium insert, which is what I did. I never tested to see if the other inserts did anything though as I didn't throw a code except for once due to running a tad bit of E85 in my tank without an E30 tune at the time.
Piggyback question-what insert is everyone/anyone using for BT? Efr 6758 catless specifically, or any other BT. Gotta get the 2 o2 monitors to ready and it's not working with the medium or small inserts so far.
I used the smallest... I have installed it on 4+ cars (all catless) and all are fine, but one.... I keep telling him to come over and we can swap it out but he is a busy man and so am I, might try the medium or it might be a fried o2 sensor.