Morocco/Mauritania border
Nothing on Maroccan side
On Mauritanian side:
55€ for the visa: you need to have the exact count in clean notes (and keep the same for the way back if you re-enter from Senegal
10€ for the passavant: vehicule entry authorization (depends on the duration of stay)
15€ for insurrance: you need to have it for cops.
Here's a picture of Cheikh, the guy I paid to help me for the border, you can do without but he's very serious and well known if you want.
The price is supposed to be 10€, I paid 5.
Diama border Mauritania/Senegal:
Before the border, you'll pass behind the national park office, the guy will say you have to pay, it seems that it's not true but after 10 min talking I paid (200 mru, 5€)
Mauritanian side:
Custom: nothing
Passport stamp: nothing (some people have been asked to pay...)
municipal Tax: 100 Mru (about 2,5€)
Senegal side:
No visa (for french...)
Passavant: 10€ for 5 days (you can extend it later in Dakar)
Didn't take insurance there, I did it in Saint-Louis
You'll have to pay to pass the bridge, 10€ (no coins) or 1000 cfa (xof) or 250mru.
About the petrol quality in Mauritania, I had no problem, but the octane rating is lower.
Except once I always got it from stations.
There were no UL fuel in El Argoub when I was there, fill up in Bir Gandouz or Nouadhibou. And of course in Chami between Nouadhibou and Nouakchott.
In Mauritania, it's not easy to find something to eat for lunch on the road, I always had, bananas, bread, sardines, nuts, biscuits... But you'll find water in every grocery.
Don't forget to take a whole lot of "fiche" for the checkpoints, you'll save time.