Your engine will be fine using Unleaded so no need for any additives
The fuel sender looks like a tall aluminium can and inside it has a float that runs up and down two very fine wires, common fault is these wires snap OR they snap when you take it apart
Just be aware it's fragile so remove the can bit with care so you can inspect the wires are intact, if there ok it might just be a wiring fault on the gauge.
A can of fuel stored up front is always a good idea if the fuel gauge is faulty.
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