If you put detergent motor oil (like what you'd put in your car) in your air compressor, it will turn milky looking. You want to run a non detergent oil. For home use, it doesn't matter if it is conventional or synthetic.
What's your budget? Some very reliable units are still made for a high price. A used industrial compressor can be had for cheap if you can wait for the right deal.
They use metal detectors to locate existing monuments, then use the plat or legal description and some math to set the missing corners using their surveying instruments.
You need a boundary survey. Have your deed with the legal description ready. The cost varies by region, terrain, and difficulty in researching.
Source: I'm a professional land surveyor