Magnesium detergents can give higher TBN for a given amount of magnesium than otherwise identically structured calcium detergents. With the sulphated ash being limited by many specifications and a desire for high TBN, some formulators choose to use magnesium, along with calcium.
Mobil 1 5W-40 TDT and Delvac 1 ESP use significant amounts of magnesium detergents too.
Is Sulfated Ash really reduced any? RotellaT 10W-30 is at 1.3%. Your average Valvoline Synpower oil is 0.93.
Rotella-T 10W-30 also has a TBN of 11.0. That's pretty fantastic right?
Would there be any downside to running a diesel oil like this in 4cyl passenger car engine?
The percent different between 1.3% and 0.93% is quite large, around 30%. That's what counts, not the difference of those two percentages.
TBN of 11 is higher than most oils start with these days. There are some with more. One example: M1 0W-40 TBN is around 12 IIRC.
Downsides to 1.3% ash? I can't say for certain. Different engines have difference "tolerance" for ash...in the piston/rings/combustion chamber regions. It's not going to make really bad things happen. At worst, it won't be ideal. At best, those regions will get cleaner in terms of carbon and varnish. You'll have to try it and see what you think of the oil.