id use Mobile 1 0W-40 way over the 15w_50. its the same oil that is sold in europe and used in some of the fastest road cars like AMG mercedes and porsche's. Also has a grat additive package and resists viscosity increase and decrease very well and exceeds ACEA A3/B3 and MB229.3/5 and Porsche oil specs.
However, another new oil that is probably better is the NEW German made Castrol Syntec 0w-30 (it says 0w-30 in red not yellow like the old one) and is the only REAL true synthetic sold by Castrol in N.A. Meaning it is made from group IV and V base oils unlike the other crappy (good oils but not for $5/qt)syntec oils that are group III. Its actually imported Castrol SLX 0w-30 and is the ONLY xW-30 viscosity oil sold in N.A. that exceeds BMW Longlife '01; MB 229.3; ACEA A3/B3 and is approved/exceeds Porsche specs for use in their engines that usually run Mobil1 0W-40 in them.
I have used Mobil1 0w-40 from like 2Kmiles on until this past sunday when switched to the new Castrol 0w-30 and I can say that i think it has more Group V esters than Mobil1 0w-40 due to the sweet aroma after use (esters do this when heated). But, you cant go wrong with either oil.
And youll need to put 3.5L including a filter change. Although, i run the Mazda 2.5L V6 filter which is larger by about a 1/3 and thus filters better and holds about .2 litres more oil. Your choice.
PS. Another oil thats out (in some areas. out everywhere by late summer supposedly) is Mobil1 0w20. Im not sure if its replacing their 0w-30 but ill bet its even tougher on breakdown. Also, remember, the first Mobil1 that came out was a 5w-20 viscosity!