It's always so easy to tell other people what they should buy, but there are two common electronic gauges that an owner of the 6.0 should own one of.
Either the Scangauge II or the Edge Insight (CS or CTS model).

Scangauge strong points are it's price ($160) and it can be programmed to read a vast number of items off the engines computer.
The downside is the number of gauges you can read while driving without having to push buttons is half of the Edge CTS (4 vs 8) and you cannot add other gauge attachments. Some also say the appearance is very 1980's and possibly cheap.

Edge strong point is it can display 8 gauges at one time on the screen (CTS model). Additional sensors can be added. A rear view camera can be added and displayed on the touchscreen.
The downside is the cost. The device itself is expensive, as are the sensors if you want to add more (although not much more or less than having to add stand alone gauges anyway for EGT's and Fuel Pressure).
Also the list of things it can read is less than the scangauge, but still decent.


Either of these devices will let you see you the battery voltage, the FICM voltage and the oil and coolant temps (so you can calculate the spread between them). You can also read codes on the fly and this will allow you to post them on here and get additional help.