Deano777
Never trust fuel gauges for ANYTHING. They are usually either completely useless, or nearly so. Flying school/club planes are often maintained to the minimum standard legally possible and gauges aren't important.
The way to do it, always, is to work out how much fuel is needed for the proposed flight, and make sure you have it (plus a decent reserve), using a PHYSICAL (visual) check. Always visually check the tank content before every flight. Never accept anyone else's word for it - not even that of an instructor! Even if an instructor says there is enough for 3 hrs, the tech log says 3 hrs, and the daily flight log says 3 hrs, still check it because all of these could be wrong (I've seen it myself).
If permitted by W&B, always fill up to the top. Then you have a known endurance (hours airborne) at a given engine power setting. Make sure you know what these are (the endurance in hours, and the cruise power setting in RPM or whatever, and the mixture setting if not full rich).
If you cannot fill up to the top, be extra careful. Regardless of how you have done your calculations, ALWAYS visually check what's in the tank before departure. I know on many planes the fuel level isn't visible through the filler cap once at or below say 50%, and on some the W&B are so severely limited that those planes often "have" to be operated below 50%, but then you are taking on a very big risk. The incident referred to in this thread was partially caused by the fact that the fuel level could not be checked.
The reason fuel gauges are usually c**p and few people seem to care is that one should not need them for flight/fuel planning, and once airborne they are no more than handy for knowing when to switch tanks. For the latter alone they ought to work but usually they aren't even OK for that; when I used to rent I used to carry a kitchen timer set to 30 mins.
Genghis
I have capacitance gauges and they are highly accurate, in fact as accurate as I can read the dials. These are the only fuel gauges which I have seen (on about 10-15 diff planes) which are actually usable.
They should not be susceptible to fuel grade etc unless they were defective to the point of falling apart. Have you any specific information that diff brands of fuel affect these gauges?