I have yet to see the thermostat drop below three bars once warmed up.
Unless you're riding in ambient temperatures below about 45?F, you won't. Even in warmer weather, if the thermostat isn't fully closing, the engine won't reach operating temperature and the ECM will keep dumping more fuel into the mixture until it does. This has a noticeable effect on fuel economy.
It has amsoil 20w50 oil in it, could that heavy of a oil drop the mpg?
The laws of physics say that it takes more energy to overcome the viscosity of a gooier oil, but not to the point where you're getting that big a drop in mileage. (The automakers are designing with 0W-20 in mind because it helps their fuel economy figures, but for them a quarter of a mile per gallon is significant.)
At any rate, put the recommended viscosity in the bike. 20W-50 isn't it and may not lubricate all of the parts as the designers intended.
How do I test if it's the upper fuel sender, or should I buy a new one?
Run the lower tank dry, remove the sender and check its resistance. When fully raised, it should read 6 ohms or less and fully lowered should be on either side of 215. (When your service manual arrives, the procedure is in chapter 22.)
--Mark