I think the fear of an oil like T6 5w40 "ruining cats" is way overblown.
Consider this:
Manufacturers typically won't acknowledge an oil consumption problem unless consumption is on the order of 1 quart per 3000 miles, or sometimes even more (1 quart per 1k mile). Yes, we all consider that to be pretty excessive, but I'm talking about manufacturer guidelines here.
So, that means the manufacturer doesn't think that burning their recommended oil at a rate of 1 quart/3k miles will harm the cats for the 100k miles that they have to certify that the emissions system should, on average, last. So what does your engine ACTUALLY burn? If its anything less than 1 quart/6000 miles, then you can use an oil with twice the concentration of "cat damaging" additives like ZDDP as the manufacturer's recommended oil, and you are still exposing the cats to EXACTLY the same number of micrograms per mile as the manufacturers approved oil at their approved consumption rate.
Now in truth, your engine probably burns a lot less than half what the manufacturer "allows." So you should be able to safely run just about any oil you want to, at least as far as cat damage is concerned.
In more practical terms, RT6 currently has something like 1200 PPM of ZDDP, compared to a Dexos-rated Xw30 which has order of 800 PPM max. So its NOT GOING TO HURT the catcons. But Dexos addresses a lot of other things, too, like NOACK. Don't know excactly how RT6 stacks up on that.