I *think* that what you actually have (unless you've modified it otherwise) is an oil pressure indicator, disguised as a gauge. A
real oil pressure gauge would not constantly read mid-scale, without fluctuation, regardless of incline, RPM, etc. The oil gauge in the (stock) Ranger is an idiot light with a "gauge" for a light.
...and, the problem is (largely) in the sending unit, not the gauge itself. The sending unit, in essence, sends "yes" or "no", and nothing in between. If memory serves, the "no" doesn't get triggered until the oil pressure drops below 6 [yes, you read that right: an oil pressure reading of 40 looks the same as an oil pressure reading of 7 on this system!]. I have seen, and perhaps it will make its way to our "How-To" section here, a write-up on replacing the oil sending unit and tweaking/calibrating the stock oil gauge to get a -real- reading.
As for your original problem, if you've already traded out the sending unit once or twice, and still see the "wobble", it could be the gauge that needs some repair work or replacement. I would recommend seeking out and employing a real fix, as I outlined in the previous paragraph.
