I have been hearing this questions since I started the game oh these many years ago......should I used oil or grease to lube my over and under? The truth is that it doesn't really matter, as long as you use one of them, and use it correctly. OK, so now you want to know what 'correctly' means.....
First, you need to understand the strengths and weakness' of the 2 types of lube.
Grease does a great job of lubricating, and does a good job of staying where you put it, but it does collect and hold dirt and grit. These solid particulates can quickly turn your grease into an abrasive compound, grinding away at your expensive blaster. If you like to use grease, you need to be diligent about wiping out the old grease, and applying fresh grease very frequently[ I suggest ever time you head to the range ].
Oil also does a great job of lubricating, and unlike grease, it holds dirt and grit in suspension and allows it to flow out of the way of moving part, thereby mitigating the abrasive qualities of the contaminates. So far, so good, but there is a downside. Oil does not do a very good job of staying where you put it.....it creeps, flows, seeps, and generally goes places you don't want it. Like on you new shirt, shooting glass lenses, or worse of all, falling victim to gravity and flowing down into your expensive wood when your shotgun is stored in a gun rack. Gun lubricating oil is VERY bad for your stock, so we don't want that happening. This is the main reason we don't recommend the ubiquitous WD40, as it is designed to penetrate and we don't want it penetrating your beautiful walnut gunstock.
Soooo, if you choose grease, change it often. If you use oil, use it sparingly, wipe off the excess, and consider storing your trusty boomstick upside down when it is put up long term in you gun rack.
I'll be following this up with a couple of additional posts regarding which lubes I recommend, and where to apply them. Before anyone gets too excited, as long as its not WD40, just about any lubricant will do.....