A funnel wont help, i had a problem with my AU III XR6, the 'no return' flap had broken off and went down to the little plastic vortex and made it impossible to fill. I tried a funnel and all it did was make a mess, and you look like a dooch with a funnel
If you dont have a blockage, the funnel may help but i think it just comes down to patience. I also find different servo's flow better than others