Hello Devo, it is very possible that the 4145 filler is a closer match to the make-up of Mercury's casting recipe and that is why they are suggesting it as the proper choice. Both of the choices that they have suggested indicate a comparable strength capability, they also suggest that the softer weld deposits will be less prone to cracking if that could be an issue.
     As far as which die-cast aluminums are weldable, most of the aluminums that I have been given to attempt to weld have not been readily identifiable so it has been a matter of experimentation. I first look to determine if the castings are used in a marine setting or some other sort of application(this can be especially important if it has been in salt water since the chlorides can have a large effect on weldability). I have found that the majority of aluminum castings that I have been exposed to can be welded with either a 5356 filler or a 4043 filler. I generally start out with the 5356, as it is of a slightly higher strength, if I encounter cracking problems I will excavate the weld back out and go to the 4043. Visual inspection of the casting will usually give you a good indicator whether the casting is of a porous nature, applying a slight bit of heat with some sort of torch will also give you an idea of whether there are any oils or other foreign elements present in the casting(if oils or greases are there they will boil out and possibly catch fire, if there are chlorides present they may turn black and show up as specks in the weld pool or at the boundaries of the weld pool and not allow the puddle to wet-out readily). In most cases if I am attempting to make a repair on an aluminum casting I make sure that the recipient realizes that there are no guarantees on the repair and I also consider the purpose of the casting and make sure that there are no safety liabilities that would come into play in the event of a failure. Hope this has helped a bit. Regards, aevald