Any possible additives to a drain cleaner I use as a source of NaOH?
The product you've described contains the following ingredients (per the ingredient data sheet on the manufacturing company website):


Sodium hydroxide
Sodium chloride
Aluminium
Mineral oil


One possibility is that the aluminium is there to provide a foaming action, as its oxidation in the presence of aqueous sodium hydroxide liberates $\ce{H2}$ gas, per the following reaction (source):

$$\ce{2Al + 2NaOH + 2H2O -&gt; 2NaAlO2 + 3H2}$$

The aluminium and sodium hydroxide would need to be kept separate and relatively anhydrous in the bottle to avoid reaction, so I wonder if the purpose of the mineral oil (which is hydrophobic) is to act as a moisture barrier and prevent mixing. If this is indeed the reaction that occurs, then the hydrated aluminium salts present are going to be at least somewhat soluble in solution, so I think it's improbable that what you have is pure sodium hydroxide. That said, if the quantity of aluminium is indeed small, this might not be a problem for your purposes.