I agree with Francisco, but I think you are talking about 'hablar de tu vs hablar de usted'. I think It's not more in Latin América than in Spain, and you can choose between le/lo (or la if female) as you wish, but, if you want a trick, use always 'le' and it'll be always right when you are using Ud. form, even for male and female.

Hi Esteban. Lo and le are not interchangable. 'Le' normally represents the indirect object whereas 'lo' represents the direct object. So if I say: "Le dije 'te quiero'" That 'le' refers to the person to whom you are saying 'te quiero' Whilst if you say: "No te lo comas" that 'lo' refers to the thing you are not allowed to eat. It has nothing to do with formal or informal Spanish. What's more, the wrong use of these two words is considered as a vulgarism, thus incorrect (that is when you use 'le' instead of 'lo' and vice versa) Sometimes it even changes the meaning of the sentence 'Le pegué' (I hit him) 'Lo pegué' (I stuck it) (Note: pegar in Spanish means both Hit and Stick)