Metal type makes a difference, as does the actual color.

Note: Non-contact infrared temperature gauges
are not recommended for use in measuring the
temperature of shiny or polished metals.
For R/C use, there are 3 commonly used emissivity
settings:
1) Aluminum: Sandblasted (standard car and
airplane engines, not anodized) set the
emissivity at 21E.
2) Aluminum: Anodized (for colored car and
airplane engines, may be powder coated) set
the emissivity at 77E.
3) Batteries Packs (with or without covering) and
Airplane Covering: Leave at default 95E.

http://www.helihobby.com/download_fi...100-manual.pdf