I use FlatOut Sportsman’s Formula in my tires, 16oz each, so one 32oz bottle per bike with 26x4 tires. I’m over 1100 miles on mostly gravel with lots of blackberry thorns, not a single flat. It’s $17/bottle on Amazon and comes with a tire stem removal tool. Takes just a couple minutes to add to each tire and lasts for ten years. No noticeable feel in the tire. Probably the best anti-flat protection available.