It is all about context. Its less a question of what is right and what is wrong. The grammar is correct in every scenario, its more about what you are trying to convey. For instance, I would say "The ingredients contained moisturize lips." Commonly you only use "may" for negatives. For instance, "the ingredients contained may cause high blood pressure." If the main intended use of the product is to moisturize lips, then you actually would only call it a "lip moisturizer." Sorry for the long answer, it is difficult to explain all the details in writing. Hope this helps!

It’s a possibility. And to correct your question * not guaranteed

“May moisturize lips” is better if it’s not a guarantee, otherwise if it will for sure moisturize you would say “this product moisturizes lips.”