Any CAT harm only comes if your engine is consuming oil. No consumption, no harm because there is no partially burnt oil going down stream to the CAT. LOts of us have been running heavily dosed HDEO oils in passenger cars for decades, and our CATs last well past 200,000 miles and the engines pass SMOG testing just fine. It's not really a worry.
If, however, your engine is burning oil at something in excess of 1 qt per 1,500 miles, you will both ash cap and kill a CAT. Been there and done that too ...
Formerly in marine engineering. In an earlier life I owned my own petroleum tank truck, and hauled for the majors and independent's.