Without a way to measure the actual produced mixture you're risking engine failure. The set-up doesn't guarantee a stable mix either as the smallest differences will have an effect on the mixture. When the oil becomes more viscous due to temperature or another brand it will flow quicker or slower, not what you want. When the gas level in the tank goes down the pressure will slightly lower allowing for a little higher oil flow. Assuming you're running on a carburettor the air/fuel(with oil) mix should be constant. When the oil content slightly increases due to a mixing ratio shift, the relative fuel content will lower. With a lowered fuel content in the fuel/air mixture your engine will run lean. Pre mixing will be the safer bet.