As I understand it, A356 is a common alloy used for many sand-cast aluminum parts. You can identify sand-cast parts often by their "rough" texture. (Like automotive cylinder heads, intake manifolds, etc.) If your aiming to repair auto and motorcycle castings, you'll come across plenty of it!
At 7% Silicon content, it has a just below eutectic mixture in it. It also has some Magnesium in it (.38%), the combination of these two ingredients make it heat treatable. And also, be aware when welding it, can make it weak in the heat affected zone. Some strength may regain over time (from "natural aging".)
According to the Alcotec aluminum filler rod selection chart, A356 filler rod is useful for welding A356 and other similar cast parent alloys. In comparison to 4043, it has a slight edge on tensile strength earning it an "A" rating for that application, versus 4043's "B" rating on same application. This may be helpful for a sand cast aluminum application where you need all the as-welded strength you can muster and is worth the extra replacement cost.
Another alloy to be aware of as an option for welding sand cast aluminum, is 4047. It is often sold as a brazing rod; and its basically a eutectic mix of Aluminum+12% Silicon. Its supposed to work well for welding cast aluminum, even though its not officially recognized on the Alcotec chart as a welding alloy for that application or any other aluminum welding application (why, I don't know.) I haven't tried it, but have heard good things. It melts at a low temperature and when it melts, its completely melted (not much of a "slushy" range, between the solidus and liquidus temperatures.)
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