When you have an 8V or 12V analog signal with little chance of exceeding 12V, a resistor divider or the common drain amplifier would work well, as you are looking just for a scaling, or impedance matching. For most automotive analog sensors, I don't know the impedance and it seems relatively non-important, so for simplicity sake I would generally use a resistor divider, followed by a clamp and low pass filter. One place this falls a bit short, is when you have a signal that can get much higher. For example, an injector driven by a voltage clamped MOSFET. The injector is 12V however the MOSFET voltage protection is often clamped around 40V to 70V. If you use a resistor divider followed by a clamp, when the clamp activates you have a low impedance, which shunts energy away from the signal wire and skews the signal being read. The issue was that we needed to look at a 12V signal with potentially up to 100V spikes.
The solution that was sought out was to use an op-amp which allows for a much higher impedance. A MCU ADC with 10k series resistor will not produce good results. However an opamp with 10k series will impedance match good enough. This meant that when the a larger signal was present on the input, and the clamp activated, the impedance was still 10K instead of 1K or less. The 10k impedance prevented manipulation of the signal, and allowed russian to work in parallel with the OEM ECU. I helped russian with the board layout for this op-amp based analog input, but I wasn't a huge fan. I prefer a much simpler approach when you know the signal, and I prefer a more universal input when you need a more universal input.
For the simpler analog inputs like TPS, ICT, ect, I prefer what I have posted in this schematic
http://daecu.googlecode.com/svn/Hardware/trunk/KICAD_Project_TRK-MPC5634_P3-P4-ETPU_IO_proto/TRK-MPC5634_ETPU_IO-board-sch_RA.pdf on page 3, top left 4 circuits. I don't like the diode clamps as they shunt energy to the + rail, which can cause the + rail to rise if there is no load to pull it down. In that design, op-amp U26B was connected to a precision 5V reference such that the 5V was push/pulled to 5V and would not lift when 20mA was shunted to the + rail by the diodes when clamping. Those circuits are looking to be safe when connect to 14V like you might see if you make a mistake in wiring and wired the battery direct to an input pin, while the signal that is normally looked at is between 0-5V. So voltages between 0-5V pass with only a little 1kHz low pass filtering, while above 12V is reasonably low impedance shunted away, limiting the input to 5.3V max.
I have what I believe is a better clamp circuit, which I plan to use instead of that U26B op-amp. I have posted that clamp here
http://daecu.googlecode.com/svn/Hardware/trunk/MCU_pin_protect_PCB_1channel/IO_protect_1x_schematic.pdf see middle section with Q1 and Q2. I have physically built that and I am trying to get in the basement to test it out, but have had issues with time constraints. I have my fingers crossed again to get a chance this weekend.
If that clamp circuit preforms as my simulations claims it will, it's my plan to spin a more refined analog board for russian. One that includes the above mentioned 4 circuits (with revised clamp circuit), plus a couple universal input circuits as noted in the second link.