Great to see your projects, keep it up!

Water will store as much if not more BTU/gal then oil, plus if it leaks not much mess.
Phase change materials store more BTU/lb than water, not much else can.
Water is cheaper than oil in most places.

Sorry to burst your bubble, this will not Kill your heating bill!-) unless you're using several dozen of them.

Air flow is best with less restrictions, holes between the cans looks small?
Restrictions will require more fan power to get a desired air flow.

The hotter the air the less efficient the solar collector will be. The hotter the collector the more heat it radiates out without heating the air (black body radiation).
Air flow and fan power are conflicting requirements, a balance is needed to be effective.

Due to black body radiation water stored in the collector will re-radiate its heat back out at night. Considering a black tank behind a single glass pane has an R value just above 1.
Storing the water in an insulated container R20+ will be MUCH better. Place the tank above the collector using the thermosiphon effect to move the water, no pump required.

As mentioned www.builditsolar.com has many inexpensive solar heating ideas.