Yes, there is a law (I can't remember the name, either), but I remember that it stipulates that:
1) your warranty cannot be voided by using an equivalent product of another brand (just be sure it meets the posted standards, e.g., classification or test criteria);
and
2) if any manufacturer insists that if you don't use the product (lubricant, filter, etc.) that the manufacturer sells, they will void the warranty, then the manufacturer MUST give you the product for free.

Thus, since Volvo isn't giving ATF 3309 away for free, you're clear as long as you use Mobil 3309 bought elsewhere.

But if it doesn't say 3309, don't use it!