I tell this story sometimes. I'll make it shorter by leaving out the where's/why's, etc.
M60 machine guns. 7.62X51. Not as dirty as an M16 but shooting way more rounds between cleaning/lubing. I ended up getting the new ones to break in and carrying one when we went to the field.
At some point I decided I need something different than the issue lubes. I started getting a 1 qt. can of PLS (think it was PLS, it was a light machine oil by the description on the can). When we got out of the trucks I'd step aside, open the feed cover and pour some PLS into the guts of the gun. Close the feed cover and we'd do whatever we were going to do (train on a range, take a walk through the woods as training, etc. I shot a lot of blanks and some live ammo. Lots of blanks with the blank adaptor clamped on the front sight and sticking down in the flash suppressor to trap the right amount of gas to make the gun function.
Every so often during the day/afternoon (no set round count I remember, it was more a feeling of how the gun was functioning) I'd open the feed cover and pour more PLS into it and then load it for what was next. Shoot it, pour lube in it, shoot it, etc.
It did not get cleaned till we went back to the barracks a couple days or a week later. The gun ran fine, regardless of the weather for however long we were out and how ever many rounds/blanks it fired (after that initial aggravating 200 rounds of break in).
When we went back to the barracks I'd pull the sling off it, carry it to the latrine/showers, turn the water on full hot, watch it till the beads of water seemed to hit the tile and bounce across the floor. Then I'd open the feed cover, open up the bipod legs and push that sucker back into the shower water so that the water was going into the guts of the gun. I'd stand there and watch the steam rise off the floor and the black crud run out of the gun. After the black crud stopped washing out of the gun I'd turn the water off, grab the carry handle and head to my room. The gun would be dry by the time I broke it down. I'd grab my old oily rag and wipe it down good inside and out. I'd wipe with dry bore patches to get the excess oil off. Then I'd put it together and carry it downstairs to the company armorer. Hetzel would jump my backside for bringing a dirty gun to the armory. No way I got that M60 clean that fast. He'd start tearing it apart to show me how dirty it was and he'd finally accuse me on not shooting the gun all week. I'd remind him of the times he'd been there when I did fire it (not as often as it was fired but often enough he'd remember and stop the accusations. He couldn't believe the gun was clean so quickly after coming back from the field. I'd have about a 1/2 qt. of PLS left after a week of training/firing. It is my belief that two things happened with all that light oil in/on the gun parts. One is a lot of the crud was washed out with the excess oil. The other is that the oil bath softened the carbon build up and helped the very hot water spray blow it out of the receiver and off the internal parts. The only thing I had to patch/scrub was the barrel.
I know, when I come home from the range these days there's some nasty looking lube on the inside of my AR15 but it wipes right off.
From the range.
After a few minutes of wiping off with a dry patch.
If I kept shooting I'd guess, at some point, the oil would be drying up and the crud would start burning itself to the parts.