I wonder if there is a minimum width that can be drawn with postscript.
gnuplot> set terminal postscript eps mono
Terminal type set to 'postscript'
Options are 'eps noenhanced defaultplex \
leveldefault monochrome colortext \
solid dashlength 1.0 linewidth 1.0 butt \
palfuncparam 2000,0.003 \
"Helvetica" 14 '
gnuplot> set output "out.eps"
gnuplot> plot sin(x) lw 5, cos(x) lw 1, sin(2*x) lw 0.18
In this code the sin(x) line is very thick, the cos(x) is thinner but I agree that the sin(2*x) doesn't look a lot thinner than the cos(x) line (when viewed with gv). Have you tried printing it, as for postscript the printer output is probably the most definitive test.
Other than that, do you really need a line that thin? On the wxt terminal I can hardly see it!