2011 Subaru Tribeca, runs great, low gas mileage , should I add lubricant and injector cleaner into gas tank?

ehh.. not sure about that yet. You may have some dirty sensors (maf or comparable) or other fuel input sensors that are telling the car to run too much fuel. Is the check engine light on or off? If it is off, I would start by cleaning the throttle body & butterfly, the maf/vaf, possibly even the idle air control sensor. There may be other fuel input sensors as well but that's a start. "not rare but not common" : If there is fuel in your oil, then you would want to address the injector(s) that may be stuck open before driving anymore as fuel will wipe the bearings clean and then you'll ruin the crank, etc.

What is your definition of low gas mileage? My 2010 Tribeca 3.6L gets 18 at best. And averaged over 55, 000 miles it is showing 17.8 MPG.