Lexus RX350 - Oil recommendation to add to Mobil 1 0W-40 "euro".

[Shel_B]

Always full synthetic, Pennzoil or Mobil 1 or Castrol Edge.
Current oil is Castrol Edge 5W-30 with Mobil 1 filter.
[...]
Never mixed brands before as never needed to.

If you've used different brands of oil, then you've mixed brands. According to Toyota specs, when draining the oil from the 2GR-FE, about 1/2-quart of the old oil remains in the block.

 

[NeoistheOne]

have heard the M1 EURO has higher amount of Calcium that tend to clog up catalytic converters over time

 

[NeoistheOne]

Advance has it in quart size, and if you have one close, Menards does as well and is cheaper than AAP.

Menard is excellent and has my favorite PUROLATOR filters

 

[OVERKILL]

have heard the M1 EURO has higher amount of Calcium that tend to clog up catalytic converters over time

You are mixing stuff up here.

Full-SAPS oils have higher amounts of phosphorous, which can poison catalysts if you have an engine that consumes a lot of oil.

Calcium has been reduced in API SN+/SP due to LSPI mitigation. This applies to M1 FS 0W-40 as well, it now has reduced calcium and increased magnesium.