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Recently changed the oil in my '89 Grad. I put in Mobil 1 15W-50 since I was able to order 10qts from Wal-Mart for under $60. The deciding factor was that it was the highest grade recommended in my manual and that it had zinc additives that older engines need.

I've also read that zinc is no good for catalytic converters, which my '89 has.

Can anyone clarify?-M5
 

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But Mad North-Northwest
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Yes, the zinc in any oil that burns in the engine slowly poisons the cat converter. However, oils had the higher levels of zinc for many years until recently. So cat degradation is not a huge problem for any individual car: it's more a problem over the entire US car fleet which is why the EPA mandated lower zinc levels in the newer oil formulations.

Given that flat tappets need some amount of zinc to protect them from wear (~1200ppm ZDDP depending on who you ask) and that cat converters are reasonably cheap and easy to replace if/when they go bad, personally I err on the side of using the oils with the older higher-zinc formulation.
 

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It has more to do with what type of catalytic converter is on the vehicle. Zinc really only effects modern 3 stage cats. The older 2 stage cats are not effected by the high levels of zinc.

Ford preformed a comprehensive study on the high levels of zinc in oil and the effects on catalytic converters. If I can find the paper I'll attach it later.
 
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