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is the zinc in the rotella a BAD idea to use because it can damage the Cat??



The zinc and the phosphorous (they go hand in hand)are harder on the cat--true. But you're really talking over the long haul. The EPA forced the automotive industry to guarantee (or at least offer a longer 'expected lifespan')the emissions systems (I think they're at 10 years 120K??), and in order to do this the zinc and phosphorous levels in the oils are being reduced.
If you live in an emissions testing area, or are trying to remain as environmentally conscious as possible, and you tend to keep your cars a long time, you might consider avoiding diesel oils (Rotella, Chevron Delo etc)

Its sorta like valve seat recession vs. unleaded fuel---you may or may not experience a problem down the road