As I understand the chemistry (and I didn't do too well with that subject in college

) the additive in oil that adversely impacts a catalytic converter is zinc. But having zinc in the oil is important for our cars because of the flat tappet design. I have been using Rotella (Shell) but I note that Classic Car Motor Oil (CCMO) has 1,600 ppm zinc while the older blend of Rotella had 1,200 ppm. Shell no longer discloses how much zinc is in their products but the rumor (Internet buzz) is that they have cut back on zinc to conform to new EPA requirements. I also found that Valvoline VR1 (Racing Oil) in both conventional and synthetic has zinc but that is more expensive than CCMO.
Just wondering ....