The oil that is recommended is from the supplier who gives the best deal on the bulk oil put in the engines at the factory. From the suppliers point of view a cheap price on 8 litres, a sticker under the hood and a recommendation in the Owners Manual has got to be worth a lot. Heck they might even give it away free. The oil has to be a suitable oil, tried and tested by the engine manufacturer possibly.
Synical maybe but money talks and it is a win win for both parties.