In my experience, finding fuel in the bigger towns/cities was a lot harder
I never ran out, just stop when you see a gas station having fuel. Usually you can tell because they dont have fire ext. or cones in front of the pump, or there's a big queue 
In smaller towns i always found fuel
Also up north to Tanzania getting fuel is easier, perhaps because fuel comes from Tanzania.
Also it's quite expensive, but fixed price for fuel at 290 kwacha
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