According to my Aladdin, I've put over 10,000 gallons in, I haven't spilled but a few drops and I haven't started any fires!
I don't think it is back pressure. My theory is that Beaver put a tight elbow in the fuel fill line that won't allow the high rate the nozzles to work. Yes, I'm suggesting another Beaver design flaw caused by a batch of engineers that never spent a day using a Beaver! I think they did it just so I would get frustrated at fueling time.
Being an impatient person, I do normally fuel both sides where the option exists. I put the nozzle in only far enough for the lip to catch and hold on the Beaver's filler neck. I put both nozzles on the first click. It is amazing to watch the $$$ meter spin.
Some nozzles won't go slow enough on the first click. So, I cut some small triangles out of 1/4 inch plywood. I can wedge them in under the squeeze lever and get any flow I want. The $$$ meter still spins at an alarming rate.
I keep the triangles in the red slot intended for the fuel cap on the inside of the fuel fill door where they are handy.
Now, if I can get Flying J to have a motorhome express payment line where I don't have to wait behind the drivers buying HoJos and chips!