Truck drivers don't add jack dammit to gas. Whoever told you that is a f#*@in idiot.
He's right! The drivers don't add anything.
But, the EPA requires all fuel grades and brands contain detergents additives. So what does that mean?
If you have to do it anyway, you might as well use it to your advantage and use it as a marketing tool.
Shell is taking advantage of a law by saying we're adding all of this out of our good graces. It's not illegal and most people don't know better. BP does the same thing. Truth is, everyone has to do it.
If your engine is running fine on 87 octane, and isn't knocking or pinging, you're probably okay. The higher the compression, the higher the octane rating you should go. But.....you got to be careful of the names and watch the #'s. In Ohio where I live, the Speedway up the street sells Premium at 92 Octane, but just 5 miles down the road and across the Ohio River in KY the Premium grade is 90 Octane. Again, legal.
Another huge thing to watch for, as mentioned above, is Ethanol.
There is a difference between pure gasoline and ethanol blends. A lot of states require the blending of ethanol into the higher octane gas to get the higher rating. This is also the major reason we don't get the better MPG numbers with higher octane gas. E10 is the tip off your getting ethanol.
Now, sitting in that class on fuel and octane ratings has finally come in handy. The sad part is I could really bore you about diesel cetane ratings.