I agree with the poster above.
Think about motor oil as just that, motor oil... its a lubricating agent designed for engines.
Despite the brands, you basically have 3 types. Std. Non-Synthetic, Synthetic Blend, and Full Synthetic.
Within that, you have to understand that the molecular construct of motor oil is virtually all the same (even more so with Synthetic). The very fundamental aspect of a lubricating chain of molecules is unchanged. What brands do, is simply change up the additives they mix into the batch to call their product unique and to try and give them an advantage of being sold. (Its all about marketing and sales really) but the competition is fierce, and people have opinions on oil as the day is long.
Find the spec's for your GW, and shop for oil despite the brand, and whatever floats your boat, buy it and use it. I may get flamed here, and other Petroleum Engineers chime in to illustrate to me the errors of my ways, but this is my opinion and observations on the manufacturing processes of lubricating agents and oils. They are all designed molecularly around basic specific structures... with a little fairy dust thrown in for kicks to help it sell.
In the case of BMW and Honda... they all have their own oils, not because they are Petroleum Engineers and designed a better product... they have their own, because its that profitable to bottle up someone elses stuff and sell it in a bottle with a logo on it. People buy into that.