EDIT: Duke pointed out below that running less than 80L is not smart.
------
For Zolder it might also be beneficial to start with around 60L (15.9 gal) and then take up to 60L again at the pit stop (+ tires). This will distribute the tire wear more evenly over the two stints.
However, assuming many on the grid take 80L (21.1 gal), you should do well in qualifying, otherwise you'll be held up by people with more fuel at the start, where you would want to try and create a lead on them.
------
Like the 1st post suggests, if you crash or pit before lap 10 you'll have to pit again because the fuel limit is still in effect during the pit stop, making it impossible to bring enough fuel to finish the race when pitting before lap ~10.
My strategy will probably be to take 80L and wait to pit until I'm out of fuel or being held up. It's very doable to leap-frog someone here if you practice the pit entrance with the two chicanes, and of course the exit which is difficult.
I doubt there would be any advantage to fuel saving in this race. Of course a viable strategy often used by top teams is to fuel save when the pack is close together at the beginning of the race. They simply let off the gas well before the corner, creating some distance between the guy ahead of you and then braking very late, gaining the distance back.
Does anybody know time-wise how much fuel equals 4 tires?