As long as it's a sufficiently viscous thixotropic(?) gel, so it resists motion even when pulled down by gravity, and remains on the outer circumference all the time, regardless of temperature, it should work.
If it is not viscous enough, then over enough time or at a high enough temperature it may pool at the bottom, and if the valve is there it may clog that up, and it will also severely unbalance the wheel until it spins back out to the outer circumference (which might not be able to happen if the temperature drops to make it viscous enough to effectively become a solid...such as say, desert daytime temps in direct sunlight, vs desert nighttime temps, which can easily be 40-50F difference or more.
If you want to experiment, you can get very soft platsil from Smooth-On, and add silicone oil to the mix until it has just the properties you want. It's not cheap, though. There are probably other things you can use, but that's one I have had experience with to make molds for casting scifi prop parts and other stuff in and from.