a possible explanation for what happened is that the MgSO4 forced more of the dmt out of solution that the NaCl so that your naptha extration was actually saturated at that temperature, if everything else was equal. Due to its saturation any drop in temp (like coming in contact with a cooler syringe) would initiate precipitation.

So it sounds as though MgSO4 is better than NaCl for that purpose. Might be worth running a few more tests being absolutely certain that there are no other variables in your equation. If you results are repeatable then you may be onto something.

Thx for posting this