What's the acceptable range of fuel trims? This could all be normal...or not.

I'm with bcred here. MAF or pre-cat O2 sensor(s) if it is a problem. Or perhaps you were getting air leakage and you resolved that by turning screws.

For the record, the partial pressure of oxygen decreases at altitude. Nothing gets 'thinner' at altitude with the possible exception of the ski "bunny" at the bar after your 9th beer.

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Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual