Avoid in winter unless it's a warm dry winter!
Some sections are in the mountain passes that get blocked with snow any time from late October to April. ( even in mid-may some years).
If you see a fuel stop top-up, is generally my rule if around 1/2 a tank level.
The same for a loo break?
I normally carry a snack or two in the Panniers/Topbox just in case! a liter of fuel in a fuel cell is probably over kill but if you have a Coleman petrol stove and a mini kettle why not? Good chippy at the time of writing on the front at Aberystwyth.