I read of a recent accident in Alberta in which three girls fell into an open bed truck carrying canola seeds and suffocated.
In that situation would it have been possible to keep breathing by using a shirt as a filter to breath through, keeping the seeds out? If so, would you be able to breath indefinitely or would oxygen in the local area run out? Could you keep getting fresh oxygen by staying on the move?
You would sink right down to the truck bed. Even if the material had zero viscosity, it would have a specific density of only 0.67 (Thank-you @Physicist137). Trying to "swim" to the surface would be like swimming with weights amounting to one third your body weight strapped on you.
Canola speeds are small. If somehow you prevented aspiration, you would run out of air pretty quickly, like immediately. The only way I can think of to possibly escape would be to "walk" to a wall of the truck bed and climb up the side, but I suspect the canola seeds would resist walking through them rather well. Good luck!