When I wrote on rec.guns, "Oil is bad for semi's. The robust cycling of the action throws the oil all over the place. Grease is better in almost every situation," I was asked, "Clint, I notice that you also sell Tetra Oil. What's the application for this? Barrels? Revolvers?"
Tetra oil, or any oil, for that matter, should only be used as a primary lubricant in captivated assemblies, or in assemblies that do not have robust movement. For example, crankcases in engines & motors are sealed, as oil is designed fly around inside them, and thusly lubricate all the moving parts contained within them. Of course, if the crankcase were not sealed, the oil would go everywhere, and drip upon every manner of thing, except that which it was designed to lubricate. The same thing applies to gas guns and recoil operated firearms.
So, use Tetra Oil in any "captivated" assembly (springs, detents, plungers, gears, screws, levers, slides, etc., but only when enclosed in a housing), or in applications where inertial energy will not bleed out its effectiveness. For areas not enclosed, use Tetra Grease, only.
In addition, Tetra Oil is a fabulous protectant due to its hydrophobicity, impregnation of the treated metal, and corrossion resistance. It also is terrific for treating barrel bores. The active ingredient, PTFE, actually gets imbedded into the metal surface, and makes cleaning up a dream. Black Powder enthusiasts are believers, among many, many others.
Rifle bores, when properly prepared and treated with Tetra Oil, have actually been shown to significantly improve accuracy in all calibers from .22 to .50 cal. Believe it or not. For documentation, contact Tetra Products directly. These tests were performed by independent organizations.
Now, one might reasonably ask, "When should I use Tetra Spray?" Another excellent question!
Tetra Spray is a cleaner and very light oil that does NOT impregnate the metal pores, thus allowing the full benefit of Tetra Grease or Tetra Oil lubrication. Remember, one of the biggest advantages available with the Tetra family of lubricants is that the PTFE particles in all Tetra lubes will impregnate the metal it comes into contact with. So, it works even when it's "gone," but only if the metal is completely cleaned before use, allowing the Tetra Oil or Tetra Grease access to the pores of the metal. If the metal is occluded with old lube, dirt, debris, the particless cannot bond into the pores.
So, use the Tetra spray for cleaning (not degreasing, that is another subject entirely), the Tetra oil for captivated assemblies, or low inertial systems, and grease for everything else.
Oh, and treat your rifle bores with Tetra Oil 1-3 times a year.
Hope this helps.