There is belief in some quarters, based on some evidence, that certain oil additive combustion byproducts may degrade catalysts. For this reason, the level of some additives, including zinc-based antiwear additives have been reduced over the years in oils designed for spark ignition engines.

I use HD diesel engine oil (API service category CI-4) in my precatalyst cars, however I continue to use SL (in process or being "upgraded" to SM) oils in my catalyst cars.

If you have a catalyst car that has very low oil use - say less than a quart in 4000 miles, I don't think a CI-4 oil will cause more rapid degradation of the catalyst, but that's just my personal opinion based on various arguments I have read about catalyst degradation issues.

The argument is somewhat slippery - something akin to blaming global warming solely on CO2 production from fossil fuels, when science does not fully understand the complete mechanism of global climate change. We know it has happened in the past, and it may well be happening now, but the bottom line is NOBODY understands why.

Duke