When adding Gauges, a list view is shown of all the potential gauges which can be added to the screen. This list "will" include gauges which may or may not be supported by the current vehicle. As per the manual:
"Once the gauge type has been selected, an expandable list of all of the potential gauges will be displayed (Note: those marked with an asterisk are not supported by the current vehicle)."
Please verify that the Engine Oil Temp gauge is valid for your vehicle. You can do this by checking for the asterisk (as stated above), or by going to the Vehicle Info screen (From the Gauge Page, select the Vehicle name in the upper right corner) and clicking on the "# Gauges" button. On the "# Gauges" list, only those supported by this vehicle will be listed.
Note: the reason that supported and un-supported gauges are listed is that the UltraGauge App has the ability to connect to more than one Blue Module which could be in different vehicles which support different gauges.