openttd.cfg:

# default setting
support8bpp = no
blitter =
zoom_min = 0

# reduced memory usage
zoom_min = 2

# embrace maximum 32bpp
support8bpp = no

# embrace maximum 8bpp
support8bpp = system
blitter = 8bpp-optimized

# embrace maximum 8bpp (try hard)
support8bpp = native
blitter = 8bpp-optimized