From memory F35 AVTUR is specified within a window of acceptable specific gravity of 0.775 – 0.840 but generally speaking pans out to be 0.803 and with the usual dyestuffs and additives has a coefficient of 0.99 per degree Celsius
There's the answer, MD80rookie. It does vary, fairly widely and is a factor, if filling the tanks is a normal operation. The fueler should be able to provide that fuel's current SG.
Most jet fuel does have an SG of .803, anyway much less than the usual answer of 6.75 lb per US gallon which is 0.81 kg/l, if my conversion is right
GF