Yes, I too find the fuel guage quite variable, though predictably so. It's very similar on my newer S8 to the old one, so maybe a design "feature". 
Slopes affect it, but especially first thing on a cold day it seriously under-reads.
For example, on Saturday night as I got home the gauge was "lowish" and reported range was 40 miles. I knew that the next day I was going on a journey that would pass one of my regular Shell "watering holes" within a very few miles so no concerns.
But starting off on Sunday the gauge showed empty and range zero. I ignored it and drove just over 10 miles to reach a convenient Shell station on the route.
I squeezed in about 76 litres, brimming the pipe, so it still had nearly 3 gallons left when I'd arrived, more when I left home! 
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2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions and extended leather. Ski hatch retrofit. Aftermarket reversing camera. DVB-T and XCarlink now removed as redundant.
2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation". (Even rarer than a D2 S8!) Oh, and a brand new engine at just under 30,000 miles on the factory one! 
Finally: gone, but not forgotten.....
1998 D2 PF S8. Agate Grey/Platinum. Every option (I think) except electric rear seats, Tiptronic steering wheel, ski hatch, towbar & dimming door mirrors.
e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course.  (internal dimming mirror added later)
1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon. Added Auto-dim mirror, Leather seats, Trip computer, Cruise control, OEM Ford SatNav with CD changer.
And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850!
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