Your temps are taken after 45 minutes to an hour then right? That's really the only way to get an accurate reading. Also, if he's being bathed in water with olive oil in it, please be sure to dry him off completely before allowing him to bask - as the oil can cause "sunburn"
Did you get the digital probe therm yet?The stick on types can be very off plus remember you want basking SURFACE temp not air temp to be 95f-110f.Your putting oil in the bath?He shouldnt be bathed in olive oil he should be ingesting a drop or two.