None of the bolts on the X are going to be standard... they're all metric
There is commercially available mass air flow sensor cleaner available at any auto parts store (auto zone, etc.) Just remove the sensor and spray it down with the cleaner to remove the thin film of oil that can be deposited by the oiled K&N filter over time, but that usually would cause rich mixture faults... The lean faults are usually caused by vacuum leaks.
Where did you have the faults read, do you have a code reader or did you use a shop? With most vehicles, if you have a vacuum leak you just spray a flamable cleaner (brake parts cleaner, carb cleaner etc.) over the suspected areas/gaskets and listen for the idle to change speed or smooth out when you hit the leak and it sucks up the cleaner. But with the BMW's the engine computer compensates for it so fast you can't catch it like that, you have to watch the O2 sensor voltages in the diagnostic tester and when they peg and stick at the higher voltage (0.8 volts) thats when you hit the leak.