I just got this new motor running a couple weeks ago and it's still in the break-in stage with only 300 miles on it since I got it running. Mods are...stock shortblock, GM 846 cam, ported stock heads, SLPs, 3200 stall, and 3.73s. I had a problem last week, while on the highway, when the check engine light came on. Code 13 came up which turned out to be a bad o2 sensor. The left o2 was stationary at 450 mv and the left BLM was maxed out at 160. I changed the o2 sensor and that took care of both the stationary readings I was getting and the high BLM problem. Now it seems that on startup only, my o2s are dropping all the way down into the 100s before finally operating properly. I don't remember this ever happening when in closed loop. Should I be concerned? It should also be noted that I did change cams but the car hasn't been programmed for the new one. I was told that because both my old and new cam have the same duration (222/230), programming won't be necessary even with the increased lift of the new cam .542/.563-vs-.533/.544