How to finance a rental ? "primary residence" rule enforced?

[Devan Johnson]

Situation: All of my assets are now liquid. Currently renting myself. Looking to buy a primary residence and 1-2 income properties.

 Because rates are so low, I would like to conventionally finance (20% down) a rental property, because this is the opportunity that has presented itself first. 

If I declare investment as such my rate  is 4.5%

If I declare the investment as a primary residence my rate is 3.8%. Obviously, I'd like the lower rate. 

Question:

What if I just take out a primary residence loan for a rental? Is this rule enforced? Will it become a problem if I try to take out another loan for my real primary residence?

Thanks in advance.