**Yes: oil type can affect how easily an emulsion forms, its texture, and how long it holds together.** Your observation is plausible, though the evidence does not establish a general rule that sunflower oil emulsifies poorly.

Whisking breaks oil into droplets dispersed through the watery vinegar or lemon juice. The final texture depends on the size of those droplets, the proportion of oil, and how effectively the emulsifier keeps the droplets from joining together. Oil composition affects this process in two relevant ways:

- **Viscosity:** oils differ in thickness, which affects how readily mixing breaks them into droplets. A thicker oil does not automatically produce a more stable emulsion.
- **Minor constituents:** oils contain varying amounts of naturally surface-active substances, including phospholipids and mono- and diglycerides. These act at the boundary between oil and water and can affect emulsification. Processing and refining change their concentrations.

There is research close to your question: a **2011 study of differently processed sunflower oils** examined how residual phospholipids affected emulsification and the stability of sauce-like emulsions. This supports the idea that the particular oil’s processing can matter, alongside its botanical source. [Mezdour, Desplanques and Relkin, *Food Hydrocolloids*](https://www.sciencedirect.com/science/article/abs/pii/S0268005X10001578)

The results also depend on the recipe and mixing method. In a **2022 comparison of olive, rapeseed and sunflower oils**, oil choice significantly affected separation, but sunflower emulsions showed less creaming than refined rapeseed emulsions. Those experiments used a different emulsifier and intensive mechanical processing, so they cannot directly predict the behaviour of your hand-mixed Dijon vinaigrette. They do show why a universal ranking would be misleading. [Kampa and colleagues, *Foods*](https://pmc.ncbi.nlm.nih.gov/articles/PMC8908978/)

**Refined canola oil—often sold as refined rapeseed oil—is a sensible neutral-flavoured alternative.** Choose refined oil; cold-pressed rapeseed oil has a more noticeable flavour. The Canola Council describes refined canola as neutral-tasting and suitable for vinaigrettes. It is a practical substitute, with no guarantee that it will outperform your sunflower oil in this recipe. [Canola Council](https://www.canolacouncil.org/wp-content/uploads/2020/09/canola-oil-good-for-every-body.pdf)

To check your observation, make two small batches with measured, identical quantities of vinegar and Dijon, using oils at the same temperature. Use the same whisk and mixing time, then compare both the initial texture and separation after ten minutes. For everyday preparation, a whisk and a slow initial addition of oil should make the result more consistent than stirring with a spoon or fork.