If you’ve heard the name HaGaon Rav Moshe Feinstein, you’ve probably heard of his stance on Cholov Stam milk . Bottom line, while he was Machmir on himself in drinking Cholov Yisroel milk, he was Meikil on others, stating that milk in westernized countries like the US and Canada were kosher due to strict government laws allowing only milk from a cow to be sold commercially. Should a Gentile try to sabotage a supply of milk, it would be too costly for him, therefore we assume that he will never do such a thing. Therefore, one may safely consume either, especially when one is in the middle of nowhere and needs to drink milk. However, a Baal Yirah should avoid even that.
A story is told about where Reb Moshe was at a certain convention and two different brands of cartons of milk (J&J and Golden Flow – thanks Reb Micha Berger ) were in front of him. He reportedly picked up one carton, looked at it, then put it down and used the other milk carton. Word spread that R’ Moshe didn’t consider one of the brands to be kosher, and therefore he put it down. Needless to say, many followers boycotted the said brand.
When word came to the president of that dairy company, that president himself came to Rav Moshe’s apartment. He asked him what was wrong with his milk? Reb Moshe replied that nothing was wrong. When the president asked about the incident of the bottles, R’ Moshe replied, “Oh that? I honestly don’t know why people are making such a fuss. I picked up one milk carton, which happened to be your brand, and saw that it was empty. I therefore used the other carton which had some milk in it! What’s the big deal?”