That's standard practice for every large manufaturing company that outsources part/supplies. If they have a vendor making "X" part for $2 a piece and another company will make them for $1.95 each, when the contract is up with the first supplier they'll change to the other vendor.

 

Of course everyone that says use brand x oil is the best because that is what they use. Don't get me wrong, there are oils out there that are better than others. I do not use HD oil, but I do use a synthetic because of oil change intervals. As long as you use an API (American Petroleum Institute) approved oil, you should not worry about your warranty. Just keep good records, if you ever do have an issue.