Hi Ken,
Check these two articles out, especially the first one with the highlighted area in blue. Hopefully, that will help you with your questions.
http://www.tomorrowstechnician.com/tt/tt80276.htm
http://www.automotiverebuilder.com/ar/ar49976.htm
-dseman
Most aluminum castings are silicon based alloys.
Therefore, a 4043 or 4047 filler rod will work.
There are some 5000 series castings, but they are rare.
The silicon based alloys also seem to run better than the 5000 series.
Let us know how you make out.
Good Luck
brande