I was told "so" is used to tell what the result was; whereas "so that" is used to tell what the purpose was. The question is that the case in informal speech? I feel a lot of people use "so" and "so that" interchangeably. In what situation, are they interchangeable in informal speech and when are they not?
"so" and "so that" are interchangeabe when they express both purpose and result. However, it should be noted that "so" will only sound informal when it expresses purpose.
- I work very hard so that I can be promoted. (purpose)
- I work very hard so I can be promoted. (purpose, informal)
- I worked very hard, so that I was promoted. (result, with "so that" being usually preceded by a comma).
- I worked very hard so I was promoted. (result)
It should also be noted that "so" is more typically used to express result while "so that" will be normally found to express purpose, unless "so" is chosen in informal contexts.
You can find additional information here: https://dictionary.cambridge.org/grammar/british-grammar/so-that-or-in-order-that