Actually a magnesium rod offers the most protection, as it sacrifices quicker. Suburbans come factory equipped with the magnesium rods. You want to use aluminum if using water sources that just go thru a magnesium "too" fast.
We keep a great set of five videos that Suburban provided our forums, explaining the maintenance and use of their water heaters at this link:
http://www.forestriverforums.com/for...es-135977.html
The very first one, copied below explains the anode rods and magnesium vs aluminum:
Water Heater Series 01: Video 01 - Suburban Anode Rod
Hope this helps