Using a off-the-shelf pressure gauge to measure water pressure
The "Use No Oil" is a warning placed on the gauge that may have application in a pressurized gas setup. The most common example being on an oxygen / acetylene gas welding rig. Any of the fittings including the manifold, meters, pressure regulators and connection hoses in such application are always used with clean non-oiled threaded fittings. The reason is that high pressure compressed gas (oxygen) can explode in the presence of oil and any possible spark that may occur when assembling the components of the welding outfit onto the compressed gas bottles.