It seems to me that your problem stems from the fact that the
arev package implements lowercase greek as italic only without providing any upright alternatives. Arev itself as a typeface does have upright lowercase greek glyphs available: http://tavmjong.free.fr/FONTS/
Adding to that, the µ character that many keyboards have is not the greek mu, it is the micro* symbol, which the
arev package also sets in italic (which it never should be. A greek mu is useful as italic and upright, the micro symbol as unit prefix should always be used upright anyway...)
If you can, switching to XeLaTeX would allow to use typefaces that are installed on the computer, which would give you access to the upright greek of Arev. In pdfLaTeX, another hacky solution would be to get the greek mu glyph from a Arev sample PDF and insert it as a vector graphic. Maybe someone else has an easier way though.
micro vs. mu
The micro symbol, e.g. for microhenry µH, is based on the greek mu, but is an independent unicode character. In some fonts, they look slightly different:
(this is set in Linux Libertine)
Insert both micro and mu (
µμ) here to check yourself.
Finally, this comment by timothymctim (Siunitx's \micro symbol has serif) proposed to use
phv (Adobe Helvetica), which looks acceptable to me:
This MWE will typeset the correct micro symbol (µ) instead of the greek letter mu. For an example of
...-micro-settings, see this answer.
Note that I changed
[utf8x] changes the keyboard µ micro symbol to a italic greek mu ...