Won't damage O2 or cats at all or at a fast rate as long as the motor isn't using oil. If it uses oil then that is true, but many people report no issues after 100k of HDEO useage on modern cars. I still have not tried it myself though but the HM oils do have more additives in them than say a "energy conserving" SM-rated 10w30 and I use them so far so good (but my motor doesn't burn a lot of oil)