Can I use a N-MOSFET to switch lower voltage signals?
Your circuit will work, but will have some problems in a automobile environment.  The "12V" in a car can be 50 V or more for short periods.  Your circuit puts this voltage minus about 5 V accross the gate and source terminals of the FET.  It won't like that.  Most FETs have a max G-S rating of 15-20 V.  Your circuit also lacks a pulldown, so the output voltage is undefined when the 12 V line goes low.

Here is a better way to do this:



This circuit will invert, but since its output is going into a microcontroller that shouldn't matter.  Generally microcontroller inputs can be arranged to be either polarity by the firmware.

The transistor will turn on when the "12V" line gets to around 6 V.  However, with the 91 k&Omega; resistor in series with the "12V" line, high voltages on that line won't bother anything.  A spike will just cause a little more base current, which won't matter since the transistor is already saturated with 12 V on the line anyway.