What year Cummins? You can hook up an analog test gauge to the 1/8" not port on top of the oil filter housing to verify. See if oil smells like diesel fuel at all, could be a sign of a bad injector dumping fuel into one cylinder. This is more common on the later models, ie common rail.

 

I went through the same thing. But, my light never gave any false readings. It continued to work as normal, regardless of what the gauge showed. I "assumed" they must have been on different sending units. If they aren't, I wonder why only the gauge went nuts. A new sending unit got the gauge back to normal