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I'll base it upon engineering input. GM, as well as every other manufacturer tests almost all of these aftermarket additives, and so far, very few will show any noticeable increase in HP or fuel mileage.
The oil I'm not sure of, it could make the existing oil slicker, cutting down on friction; however, the modern engine is designed to have as little friction as necessary to increase HP and fuel mileage.

However, the fuel injector cleaner, that they want you to run in each tankful is ludicrous. Read my previous post, fuel is not that dirty, injectors are designed to not crust over from spray back of fuel as they did years ago. So, why would you have to run it all the time. In addition, if you want to keep the injectors clean, run one of the Top-Tier fuels in the tank, and these Top-Tier fuel companies use larger than required amounts of cleaning additives in their fuel, so the chance of an injector clogging is almost impossible.
The Top-Tier fuel requirements are set by GM, Honda, BMW and two more companies, and include Shell, Chevron, Conoco, and quite a few other companies.

It's your money, and if you feel like doing it, do it. Keeps the economy running.