Hi Wiggy,
have you ever added even a little Racing Fuel or E10 or E85 ? Racing fuel is EXTREMELY corrosive, whereas Ethanols are just less corrosive than Racing Fuel.
Adding even small amounts of these guarantees stripping the inside galvanised lining of your fuel tank, which turns the fuel greyish milky.
If the above is not the case, and you suspect condensation or sabotage from someone adding water to tank, drain tank completely and allow a 1 litre sample from the beginning of the drain to settle for several hours or overnight. Then do same from the last of the draining. If poss, do same from middle of draining. Examine for "separation" as fuel is lighter and will "float" on water.
You should also try adding about a litre of Methylated Spirit to every full tank to make any abnormal moisture mix with the fuel. Half litre for half tank. If this clears up the problem, continue with half litre Metho per tankful as a preventative measure.
Is it possible you live in a very humid area ?
May help to ask other clients who buy fuel from same Servo you do, if theyve had any problems.
Even ask the Servo Operator if he has had any complaints.
Try getting fuel from ONE ONLY source till you nail it.
I've never heard of any Valve Saver causing any problems, but then I rarely use it as more than one industrial chemist has told me it's no better than SUPER OUTBOARD 2 STROKE OIL , which Ive been using with 100% success for more than 20 years in ALL my Carby and EFI cars........all 12 of them !
Good luck
Bernie