2007 Other GMC Models Changing Brands of Oil Multiple Times

You are correct and the service manager is correct. The only way to get data on the different additives that different oil manufacturers use is to write to them and ask for a list of additives. They will also sent you the %s of what additives they use.
I can go on and on and on about oil. But to keep it simple, there are a few things everybody should know about oil. First I want to say I am not a spokesman for any type of oil, I can only tell you what I've seen and what I know.I have worked on 800 horsepower alcohol injected race car engines, 3 horsepower grasscutter engines and everywhere in-between. BY FAR the best oil to use in any internal combustion engine is Moble-1 SYNTHETIC period.
The reasons for this are these---oil has many jobs inside of your engine. Lubricating is one that everybody thinks is the most important--it is NOT. The single most important thing oil does inside of an engine is, oil transports contaniments to the oil filter--these contaniments include any dirt, small pieces of metal, bearing material, etc. This debris must stay suspended in the oil so it can be filtered out by the filter. This is why you must always change the oil and filter together! Many people make the mistake and think that dirty oil is bad. In fact the opposite is true. Dirty oil is an oil that is doing its' most important job--getting dirty!
The reason oil makers put additives in is because some think that different additives are better than others. Moble-1 Synthetic has the perfect additives--trust me on that. Whatever oil you use I do suggest that you use synthetic. The reason is because companies can put more additives in synthetic that they can put in conventional. In my opinion--the more additives the better the oil. I'm not so sure about regularly switching brands though. As for the Service Conultant, ask him/her what the viscosic inbeddiment is for the different types of oils they are using. Viscosic inbeddiment is the ability of oil to carry contaniments to the oil filter. I bet after that you will get the same type of oil.
Oil is like gasoline (except shell gasoline). All gasoline comes from the same -physical- pipe. What makes exxon gas different from sonoco gas and valero, etc- is the amount and type of additives added to the actual tanker truck that delivers the gas to the gas station.

BTW--Shell gasoline is the only company with its' own pipeline--and a new Hummer deserves Moble -1 Synthetic oil.

One more thing. Oil is the blood of the heart of your vehicle--your engine. Would you use different types of blood in your heart? I hate Service Consultants don't you?