Update: bought a smoke machine.

Smoke tested:
Intake - no leaks
Vacuum booster - no leaks
Exhaust - no leaks

Then I drove it about 10 miles and was able to reproduce the lean condition.

This time however I noticed that bank 1 was lean while bank 2 was still looking ok.
Drove it some more and bank 2 started acting up again so both banks were now lean.

Drive it back home and did a smoke test while everything was good and hot.
Still no leaks anywhere.

Does the car go into closed loop one bank at a time?
If it goes to closed loop on both banks at once, sounds like it can’t be the fuel filter/regulator.

It may either be the fuel injectors or maybe the oxygen sensors.

__________________
2005 X5 3.0 6-spd.