IIRC, your lungs extract more oxygen from warmer air, and when you breathe in through the nose, it is warmed more on the way down to them. (Contrast with: Outside, colder air temperatures tend to have more oxygen in the air than warmer air temperatures - but your body will still appreciate that richer-oxygen cold air being warmed before it hits your lungs).
As for your biking no, there's no set resistance that matches roads, and we dont' know who made your particular bike anyway. Resistance 8 on yours might be the same as resitance 2 on mine. Different manufacturers...
Use the RPE to determine how hard you're working when biking outside, and try to match "how hard that feels" when using the exercise bike.