Oil injection tank..... the oil injector pump is under the plastic cover on the front of the clutch cover (right side of the quad) and the line goes up to the right side of the carburetor throat on the engine side of the slide.
The small red light on the back of the headlight is the oil level warning light. It should glow for a second when you first start up the quad and then go out. If the level in the tank gets low, the light will come back on telling you to refill. The cap to refill is under the back left corner of the rear plastics, under the seat. Make sure you clean around it well before removing it. Any dirt in the tank can find its way to the pump and block it.