It true, but will not have enough effect on your consumption to measure!!
"Specific gravity" effects the volume of fuel but unless your filling up massive amounts it will make no difference. Like the 50000 litres we put on Airplanes (and we only use the SG to get the weight acurate).
Once your a few feet down the air temp/sun will have no effect on the ground temp. (This is how Ground source heating works).
Fuel shouldn't evaporate out of your tank if you have a fuel cap on.
Etc
Etc...
If you go to these extremes to save fuel you will drive more economically and will notice your MPG go up
