Sonic's own Summer 2026 ingredient guide shows the fryer shortening and the bun oil are both soybean oil, and even the beef patties carry hydrogenated soybean oil, so seed oil is essentially unavoidable here. 92 of 106 food items also carry an additive, and the only 13 items clean of both are condiments, fruit add-ins and plain produce.
Sonic publishes a real per-item ingredient statement guide (SUM26, 7 pages), and it settles the question. The Fryer Shortening is listed outright as "Soybean Oil, Hydrogenated Soybean Oil with TBHQ", and there is a separate Bun Oil, also soybean oil with TBHQ, that goes on the buns - so a burger and an order of tots are both seed oil before you add a single topping. All three beef patties (Burger Patty, Jr Burger Patty, SONIC Smasher) carry hydrogenated soybean oil inside their encapsulated salt. Of the 106 non-drink items, 43 name a seed oil outright and 92 carry an additive. Only 13 are free of both, and every one is a condiment, a fruit add-in or plain produce - mustard, honey, apple sauce, lettuce, tomatoes, diced onion. Dyes reach past the slushes into the food: the Brioche Bun lists Red #40, Yellow #5 and Yellow #6, and both the Zesty and Signature Cheese Sauces list Yellow #5 and #6.