I have the diesel version of the same car, and been using FIAT 9.55535-S2 oil in it, because of the higher HTHS (fuel dilution etc, which is also possible on the Multiair).
I used Castrol Edge FST 5w-40 turbo diesel, but I find this oil gives off a lot of fumes, when you open the filler cap and it was even running down the MAF into the air filter and onto the filter housing.
So I've been searching an equivalent also. I'm going to get Total quartz 9000 energy 0w40. Slighly higher in Saps than a C3 or C2, but still qualifies as an A3/B4 and I believe carries MB 229.5 and BMW LL-01. PAO/ester blend, not hydrocracked and gets excellent user reports here on the forum. so close to a w40, the engine won't know the difference once hot. Cheap aswell, you could do 2 oil changes for the price of 1 specialty oil change, and 3 or more compared to dealer oil prices...
also look at Elf Excellium Full-Tech 0w-30. Exact same oil but older name.
Doesn't sludge get formed when the oxidation stability gets depleted? so the answer woud be to use a higher saps oil or more frequent oil changes, or both....