I wanted to bring this back from the dead with some new info reguarding the 161, 168, 194 bulb #'s. They are all interchangeable except for the light output.
This is how Autometer sells their sockets for their gauges:
Auto Meter | 3220 Bulb & Socket No.
161/2.7 Watt
194/3.8 Watt
168/4.9 Watt
For Use With 2 1/16" Auto Meter Gauges-Plastic Cases
So in terms of brightness (and HEAT if you’re using incandescent bulbs), 168 is the brightest and hottest.
Heat isn’t such a factor with LEDs though so you can probably just get the brightest possible.
Info about converting to LEDs here: http://www.superbrightleds.com/carbulb-notes.htm