The tank needs to be lowered down and the top seals replaced.
It could be something in the fuel-sending unit or a float has leaks which causes the fuel to remain at the bottom. The latter signals the sensor to send the warning.
If the fuel leaks from the tank only when the tank is full, the filler neck is probably cracked, or if made of rubber, the clamp may be loose and not sealing correctly, also the rubber may be cracked.
I switched to full synthetic engine oil in 2001 and have no oil leaks on my 1995 Ford What makes you think you will have leaks? And if you think you will have leaks then why would you use synthetic? You will have no leaks that you would not have with conventional oil.
It's lean. Check for vacuum leaks, and make sure the fuel supply is functioning properly.
The sending unit is either defective or it is hanging on something in th tank.
the gas leaks out
NO additives are recommended by any good mechanics. Only the full repair is best especially whem dealing with coolant leaks. Usually intake leaks are from years of corrosion the coolant will turn to acid in most oif todays cars if not treated you will have much internal damage. the external leaks will only show up when damage is done. Dont write off the vehicle it ,will be much cheaper to repair than pay the extra cost for insurance ,interest loss of value for new cars.....
If the guage is registering full all or when under half a tank it is a faulty sending unit. Cost is in the hu8ndred to repair.
Depending on who you talk to everyone has their own tricks. In my opinion, the liner should be replaced AND retrofitted to a full wall vinyl liner. In my experience there can be too many problems with Low Hung Liners.
You need to check the power and ground at the fuel tank. If that is okay the float is most likely bad.
Is it over full?
By doing a full tuneup; changing sparks, wires, distributor cap/rotor, changing the air filter, the oil, flushing the transmission and the radiator. From there you should repair any engine leaks, replace any old vacuum hose (all of the vacuum hoses), and put a fuel system cleaner in your gas tank. I recommend Gumout with Regane as a good cleaner.