**Yes, oil type can affect an emulsion—but there is no general rule that sunflower oil emulsifies poorly.** Your observation is plausible, although it could reflect the particular oils’ processing as much as their plant sources.

Whisking breaks oil into droplets; mustard helps keep those droplets from joining together again. Oil composition affects this process, including the small amounts of naturally occurring surface-active substances. A study comparing differently refined **sunflower oils** found that oils retaining more phospholipids had lower oil–water interfacial tension and formed stronger interfacial films. So two bottles labelled “sunflower oil” can behave differently. [Study in *Food Hydrocolloids*](https://doi.org/10.1016/j.foodhyd.2010.07.019)

However, **forming an emulsion and keeping it stable are separate questions**. A controlled comparison of olive, rapeseed and sunflower oils found no significant difference in initial droplet size, but did find differences in separation during storage. Sunflower oil performed well in that experiment. These were laboratory emulsions using a different emulsifier and much stronger mixing than yours, so they do not establish a ranking for hand-whisked vinaigrette. [Comparative study in *Foods*](https://pmc.ncbi.nlm.nih.gov/articles/PMC8908978/)

If your olive, walnut and pumpkin seed oils are unrefined while your sunflower oil is refined, their retained minor constituents are a **possible explanation**, rather than a demonstrated diagnosis. Mustard quantity, oil-to-vinegar ratio and mixing also affect texture and stability. [Mustard-vinaigrette study](https://www.fortunejournals.com/articles/physicochemical-and-flow-characterization-of-a-mustardvinaigrette-salad-dressing.html)

For a neutral alternative, **try refined canola/rapeseed oil**. It has a mild flavour and is suitable for salad dressings; cold-pressed versions can have a more noticeable flavour. There is no guarantee it will emulsify better than your sunflower oil. [Canola Council’s culinary guidance](https://www.canolacouncil.org/about-canola/oil/)

A useful kitchen test is to split one vinegar–mustard mixture equally between two bowls, then add equal quantities of the two oils using the same whisking technique. Compare both the immediate texture and separation after 10–15 minutes. For more reliable results with either oil, whisk vigorously and add the first portion slowly, until the mixture looks uniform, before adding the rest.