In some cases, I don't think it makes a big difference with modern firearms. For example, most of the duty grade semiautomatic pistols can run reliably in the worst of conditions. Ironically, one of the few things that can cause them to malfunction is excessive lube
The following is
my opinion, which I have based on: reading tests, reading on BITOG, examining MSDS and other data on LD50, examining (the limited) data on variable friction coefficients-->wear reduction & wear outcomes (most originating from the US Armed Forces), conducting my own testing, and then using the product for an extended period of time. I have come to believe that, while many of the all-in-one products do a reasonably good job, if someone wants
the best anti-corrosion protection, the best wear reduction, and the smallest amount of lube-induced fouling as possible, than it will take two different types of products***.
For rust prevention, long term storage, and for metal that lives in a humid area, I use a partially or fully "dry" lubricant. This type of lubricant, by design, favors other properties over wear reduction. I've been using Corrosion-X because of its low toxicity. One treatment will last many times that of a product like Rem Oil...I'd go as far to say Corrosion-X will provide at least 10 times the duration of Rem Oil.
However, these thinner lubricants generally offer a limited amount of wear protection and a limited amount of thermal resistance. Corrosion-X in particular is very limited in terms of friction reduction. When I want to reduce metal-on-metal wear, or know I need a lubricant in an area that will get piping hot, I use a premium grease. Not the old stuff that gunks up guns and takes years to get off, but one of several dry grease products, most of which are fluoropolymer greases and originate from S.T. DuPont. This type of grease I find to reduce fouling even better than most dry liquid lubricants. It does NOT migrate, it maintains its properties within an almost 1,000 degree temperature range, and it endures. For example, if I use this grease on a Mk 3 and don't fully clean it for a few years and shoot at least several thousand rounds, upon detail stripping this grease will still be in the exact location I applied it, and it will still have the properties it should have. With extremely cold weather shooting, this grease ensure flawless function in semiautomatic rifles. There are other types of grease on the market that more or less do the same thing as listed above, but my experience with them is limited.
I have also found that some products actually cause functional issues and can make corrosion worse than doing nothing at all. For example, I've found that Militec-1 provides no rust protection (in some cases, it can cause rust), provides limited wear reduction that's no different from other liquid lubricants, and it promotes fouling more so than other options. So I recommend trying something else.
***The Wildcard - I recently got a hold of some dry lube from Hornady, and I am
very impressed with it. I think it provides rust protection that is just as good as Corrosion-X and Eezox (maybe even better), BUT it provides much better wear resistance than other dry lubricants I have used. It also cleans surprisingly well and it does an outstanding job in reducing fouling. So this one might really be the all-in-one home run.
Just how important is wear reduction with firearms?
I'm struggling to quantify this question. Most modern firearms are made to have very solid endurance, and many are built to survive the worst of conditions. With guns that see a ridiculous amount of rounds (like a Ruger Mk 3), I've seen the grease reduce wear by quite a bit. But what about guns not used nearly as much?
