Do I need to season an old cast-aluminum pan?
Yes, cast aluminum should be seasoned.  Here are some instructions from cookingforengineers.com:



How Do I Season It?


Wash the cookware with hot soapy water.
Dry it and then coat it thoroughly with vegetable oil. The easiest way is to pour the oil onto a paper towel and work it well into all the surfaces.
Put the well-coated cookware into a 250 degree oven and leave it there for 2 hours.
Never use scouring pads or detergent on cast ware. Simply wipe it out using a damp cloth.
If food starts to stick to the cookware, just season it again.


How Do I Look After It?


Repeated washing in a dishwasher will strip off any seasoning, can cause discoloration and is not advised. Remove the staining by boiling something acidic like tomatoes or apple peelings and then re-season.
Don't leave it to soak in soapy water
Don't use steel-wool pads to clean it
You can use non-abrasive cleaners or a paste made with baking soda and water. Use either of these with a gentle, synthetic scourer and your sheet or cast aluminum will shine!




Here's one more tip I learned the hard way.  Don't use aluminum cookware on glass ceramic stovetops!  The aluminum can permanently discolor the glass, or perhaps even fuse to it.  I've never actually seen an aluminum pan fuse to a stovetop before, but I did permanently mark my stovetop the first time I put an aluminum pan on it.