My DT2 hasn’t ran for a few months because I was doing some leak down testing and replacing some parts. I am wondering since I disconnected the oil injection tube while testing if I should premix a gallon to get it going again to account for any air gaps in the system? Maybe I’m overthink it tho 

You can bleed the system.
Details will be in the manual, it's very straight forward.

Yes Yamaha manual says you must bleed the system whenever you take a line off. I think you’re right to think about this since oil in the system is pretty important! Remove the bleed screw on the pump and be ready to put it back in once a steady stream of oil comes out. It will come out fairly slowly unless temp there is hot. Put the screw back in and tighten. If your pump has the wheel then turn the wheel in the direction of the arrow on it while with your other hand rotating the pump pulley out to max pump stroke. You’ll have oil up to the carb in no time. I just put my 78 DT400E back together and no wheel on that pump. Manual says (after bleeding the pump) start bike and rotate pump pulley to max stroke by pulling on the cable at the pump and run motor at 2000 RPMs for several minutes. So either method will work. Of course look for that telltale blue smoke and you’ll know you’ve got it.