That's correct, the new 229.51 Mobil product is not the same as the T&SUV (AKA Turbodiesel Truck) oil even though they're both 5W40. You've got to keep in mind that the Benz oil specs and API specs are mutually exclusive. The spec can be one or the other, but not both.

I suspect this new Formula M 5W40 is going to be dealer-only. It's a very focused product and there's no reason to think you're going to find it at Wally's Mart.