I would like to chime in here. Hopefully, this will help someone. I can finally offer some advice back on a topic that I am fairly well versed in. I was a product manager for the world's largest pressure gauge manufacturer for 8+ years an did a fair amount of gauge root cause failure analysis and application advice and troubleshooting. All the suggestions in the thread are good ones. Please allow me to add a few more details in the hopes that some others with similar, but different problems, will be served.
A pressure gauge is a simple device. The pressure acts on a "bourdon tube" It is very similar to a New Years Eve Party Noise maker. You know, the thing we all blow on, it unravels and at the end, makes a noise! Well, the pressure gauge and bourdon tube do the same thing, only it moves just a slight distance. Otherwise known as bourdon tube tip travel. This small movement is translated by a simple linkage that acts on a segment gear. The segment gear moves due to the bourdon tube tip travel and meshes with a small pinion gear that is attached to the gauge pointer, or needle.
There are three main gauge "killers". They are Heat, Mechanical vibration and pressure pulsation. Our gauge installations don't have enough heat to damage the gauge, but, they do have plenty of mechanical vibration and I am sure, to some extent, pressure pulsation.
The most widely accepted industry method for controlling mechanical vibration is to liquid fill the gauge with a case fill fluid. This envelopes the entire interior working parts of the gauge in a lubricating and vibration damping fluid, typically glycerin or silicone. I have yet to see an aircraft mechanical oil pressure gauge that is case filled, so, my first thought is that any gauge with any fair amount of hours on it has it has excessive wear on the linkage and gear teeth, allowing the indicating needle to just sit there and act erratically. So, in many cases, especially with a high time gauge, it is not the oil pressure acting erratically and you are not experiencing engine issues. You can take the gauge apart and carefully inspect the linkage and gear teeth for wear, damage or looseness. If you find any, this may very well be the culprit of the needle vibrations/fluctuations.
Now, let's address the possibility that the gauge mechanics are OK and there is a little oil pressure fluctuation being picked up by the gauge. The most widely accepted industry standard for simply and cost effectively controlling pressure pulsation is by adding a "restrictor" to the gauge pressure entrance and throttling down the pressure to the gauge. A restrictor is a plug with a very, very small hole in it. It's kinda like making a big group of folks all go threw a revolving door one at a time. This is a very effective method for smoothing out pressure pulsations and, as already mentioned in this string. However, just to be clear, I believe most of the restrictors that we all "should have" in our oil line system somewhere (hopefully close to the block) serve a much more important benefit.....they are there in case there is an oil line or bourdon tube failure or rupture, which is entirely possible. This is disastrous in flight as you all know. Oil will be literally pumped out of the engine. With a restrictor in place, the oil loss is slowed way down, giving us crucial extra seconds or minutes of engine run time before catastrophic engine failure. So....restrictors are good and a must have. Please make sure your mechanic verifies you have one at your next shop visit or annual.
I hope this has been helpful. On a side note, if your gauge is found to be tired by looking at the internals and you verify you have a restricitor, you can add life to the gauge and calm down the needle fluctuations due to mechanical vibrations by adding some very high viscosity silicone grease to the gauge movement and gear teeth (sparingly). Try McMaster-Carr for it.
Good luck and please feel free to ask for help on any pressure gauge questions, I will be glad to help! Safe Flying!!
