The phrase "top tier" has absolutely nothing to do with octane. Even the most off-price stations have the octane ratings posted on the pump.
"Top tier" is shorthand for "high quality name brand" like Exxon Mobil, Shell, etc. Unlike the bargain brands ("Joe's Great Gas"), top tier fuels tend to have a better additive package (detergents and anti-foaming agents). These additives cost money, and are largely responsible for the price difference between top tier and cut rate brands.
The brand is only part of the equation. You also want to buy fuel from quality, high volume stations. Joe's Great Gas may not sell much premium or mid-grade fuel, so their fuel may sit in their underground tanks longer, picking up moisture. Moisture in the fuel can wreak havoc with modern engines, and the higher the ethanol content, the easier it is for the fuel to pick up moisture.
From the Owner's Manual:
Super Premium Gasoline/AKI 91
This gasoline is highly recommended. However, you may also use gasoline with less
AKI. The minimum AKI Rating is:
> BMW X5: 87
> BMW X6: 89
If you use gasoline with this minimum AKI Rating, the engine may produce knocking sounds when starting at high outside temperatures. This has no affect on the engine life.
Do not use gasoline below the specified minimum quality, otherwise engine damage can result.
Use high-quality brands
Field experience has indicated significant differences in fuel quality: volatility, composition,
additives, etc., among gasolines offered for sale in the United States and Canada. Fuels containing up to and including 10% ethanol or other oxygenates with up to 2.8% oxygen by weight, that is, 15% MTBE or 3% methanol plus an equivalent amount of cosolvent, will not void the applicable warranties with respect to defects in materials or workmanship.
The use of poor-quality fuels may result in drivability, starting and stalling problems
especially under certain environmental conditions such as high ambient temperature and
high altitude. Should you encounter drivability problems which you suspect could be related to the fuel you are using, we recommend that you respond by switching to a recognized high-quality brand such as gasoline that is advertised as Top Tier
Failure to comply with these recommendations may also result in unscheduled maintenance.
2014 535i MSport Space / Black, ZCW, ZDA, ZLP, ZLS, ZPP, H-K and 704
2015 X1 28i xDrive Alpine / Black, xLine, ZCW and Ultimate
2015 X1 35i xDrive Black / Black, ZCW, Ultimate and Xenons
2014 328i SportWagon
2011 535ix MSport
2011 X5 35D
2008 ///M3 Vert
2008 X5 3.0
2007 X5 3.0
2006 X5 3.0
2006 550iA SP
2003 540iA M-Technic