I find a voltmeter, in conjunction with the ammeter, very handy. Hook it up so that it comes on with the ignition switch. When you first turn on the ignition it will tell you how good your battery is. You can view the voltage when cranking the engine to see if the battery is good under load. While you are driving you can monitor the battery voltage. If you have an alternator it will come up to a fully charged state, 7 or 14 volts, and the ammeter will be at zero. My battery shows about 8 volts when I am cranking the engine (12 volt system).
My alternator failed once while I was on a trip to a friends house. The ammeter showed a slight discharge, but the voltmeter was a spot-on clue. I monitored the voltage while I completed the last few miles to make sure that my ignition system was going to make it. The alternator was changed out at my friends house.
The oil pressure is important, of course, but I find that I am looking at the tachometer a lot more than the oil pressure gauge. I have an overdrive so I can keep the engine rpm in the right range for the conditions, such as climbing a grade or cruising down the highway.