Any issues running 98 octane in 4.4 X5?

[JCL]

Originally Posted by m5james

I know there are conflicting opinions about using higher vs lower octane, but using the cheap or too low of an octane in BMW's has never served me well. This is my .02, but using the highest recommended octane will net better gas mileage as opposed to using lower octane in an effort to save money because you will get worse gas mileage, making the cheaper stuff a wash anyways.

That is a blanket statement, and not entirely correct. The best fuel for any vehicle is the lowest AKI rating that doesn't cause the engine management system to adjust the timing to prevent knock. That may be the same as the recommended fuel, but it is unlikely. Fuel quality and specifications in the real world follow a bell curve, so the manufacturers specify a fuel that will nearly always be OK to use. That puts the specified fuel above the average required AKI, if you believe in bell curve distributions.

It has nothing to do with saving money, to me, since the savings are only pennies. It has to do with using the best fuel for the engine, and that isn't necessarily the highest AKI, although it sometimes is.

The safest and easiest is still to use what the manufacturer recommends, but using a lower AKI won't reduce power or mileage until you get low enough that the timing is retarded by the engine management system. That is why my vehicles always ran on 89 AKI (until I got the 535i with the turbos), as it was better performing in winter (we don't get high temperatures here). 89 solves some hard start problems over 91 AKI, in cold temperatures. Best mileage I ever got in the X5 (and the Z4) was on 89. However, we have high quality fuel here, and you may need to get to a pump label of 91 to get the same real world AKI, depending on where you are purchasing it in North America.