I'm kicking around running the M1 0w-40 just to see how the car runs with it. I know it's full SAPS with higher ZDDP etc. should I be concerned about damaging cats? The engine does use about a qt in 6k miles
It has more than your typical 0w20 but you are talking about 1 OCI just to try it out. I would say in the long run it won't matter. Running it every OCI probably won't either and might even decrease your consumption.
If that 3.0 has HLA don't do it. HLA at times require specific hot bleed-down rate to get proper low rpm torque. At most go to a light 30 like Motul econergy (well it USED to be a light 30 at KV100 under 10).