You ask, "does the 'new formula' contain an additive pack that is harmful to catalytic converters?" The answers are no, yes, and I don't know. The additive pack can't touch the catalytic converter unless the engine is burning oil. Saab engines generally don't burn much oil. So the answer is generally no. But if a specific catalytic-equipped Saab is burning oil, then yes, the additive pack probably is harmful. The "I don't know" answer springs from the fact that Shell doesn't disclose information about the T6 additive pack. If you want to know the secret ingredients then the best you can do is look online for a recent (2017 or later) virgin oil analysis (voa) of T6. The jug of T6 you buy may or may not have the same package depending on things like whether it came from the same batch, did Shell change the formula, and did the testing lab run an accurate test. The last T6 voa I saw was done in May, 2017. For anti-wear it had 1049 parts per million (ppm) of phosphorus, 1095 ppm of zinc and 198 ppm of boron. Phosphorus is the chemical that damages catalytic converters. For detergent it had 2079 ppm of calcium and 93 ppm of magnesium. It is because of the 1049 ppm of phosphorus that I say yes, if your gasoline engine is burning oil then T6 probably isn't good for the catalytic converter. T6 never was good for catalytic converters, not even when it had gasoline engine ratings.
Shell dropped the API Service SM, SL, SJ and SH ratings of Rotella T6 in response to a change in the API's guidelines. Shell's move was a marketing decision, a product decision and a political correctness decision. Their product decision was to continue making a heavy duty motor oil named Rotella T6 aimed at the diesel market. Their marketing and political correctness decisions were to steer consumers with gasoline engines to other oils they offer for late model gasoline engines.
API forced such decisions by requiring makers of the CK-4 diesel class oils who also wanted S_ ratings for gasoline engines to formulate the additive pack with no more than 800 ppm of phosphorus. For Shell that would require turning Rotella T6 into "Rotella T6 Lite" from the perspective of most owners of diesel engines (but not of most owners of gasoline engines). Diesel engine manufacturers like Cummins, Detroit Diesel and Ford would respond to "Rotella T6 Lite" by telling their customers to stop using "Rotella T6 Lite" in diesel engines that require a heavy duty motor oil.
Oil unfortunately requires some attention because the rules keep changing and oil makers can make significant changes without disclosing it on the bottle. Consider for example the popular Mobil 1 10w40 High Mileage. With Mobil 1 we don't have to guess about its phosphorus content because Mobil 1 from time to time publishes Oil Product Guides that include that information. If you have a copy of Mobil 1's Product Guide that was updated Sept. 2014 then you can see that 10w40 High Mileage had 1000 ppm of phosphorus, 1100 ppm of zinc and an API rating of SN. If you have the Mobil 1 Product Guide dated March 2017 then you can see that 10w40 High Mileage now has 800 ppm of phosphorus, 900 ppm of zinc and an API rating of SN.
To my engine which was designed for SG oils (typically containing 1200 to 1300 ppm of phosphorus) that is potentially a significant change. Some older engines have problems with valve train wear when phosphorus drops below 1000 ppm. Even late model Ford Powerstroke diesel engines have a problem with low phosphorus levels. I don't know for a fact that my 22-year-old B234 engine requires 1000 ppm of phosphorus. I won't switch to Mobil 1 10w40 High Mileage or any other SN rated oil just to find out. I'm staying with Rotella T6 and the expectation (not the fact because Shell won't say) that it has 1000+ ppm of phosphorus. This is not my final answer because the game keeps changing. For all I know Mobil 1 10w40 High Mileage and a lot of other SN synthetics with ACEA A3/B3 ratings are still great because either my engine isn't that sensitive or the oil makers have offset the reduction of phosphorus with an undisclosed increase in boron or some other catalytic-safe anti-wear chemical.
It's complicated because of the lack of detailed additive pack information. The answer that is "right" for me may not be the only right answer, and it could be wrong for a lot of other Saab owners.
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