Most of us would rather spend the time it takes to clean a gun on the Internet asking what the cleanest powder is.
Ha ha! No joke, it REALLY makes a difference! The H335 powder grimes up my 6.5G AR much worse than an M4 using M855 (and whatever they use in it). You can really tell in an AR, that DI is real fine for simple function and accuracy but really funks up the action.
I like to clean mine after, but when I take several to the range and stay all day, I just don't get around to it. I SHOULD at least wipe them down real good, I can do that pretty fast, and then hit 'em with the dental picks later. Besides, unless you clean them while still hot (fastest way) then wiping them down and "soaking" the action until you get around to it is the best way. Hoppes will soak into the grime and makes the carbon come off much easier if you do this. Running a mop down the bore with a moderate amount of CLP isn't bad either, but too much can actually trap water and make it rust.
As for changing oil in my truck, it gets changed about twice a year. Wife's car too. These are '05's and they run fine, the truck is still like new, but only has about 25,000 miles on it. I use the expensive Mobil 1 full synthetic. If you run this stuff, you can run it for quite awhile before it breaks down. It doesn't seem to go bad near as fast as regular oil. Where regular oil would be BLACK GRIME in 5000 miles, synthetic is only starting to look like 1000 mile regular oil.
I've read more than once that synthetic can be ran for, no joke, up to 15,000 miles. I first changed it at 3000, then that seemed like a waste, then went to 5000, then went to about twice a year. Works out to be average for the car, overkill for the truck, but it needs to be changed at least bi yearly even if you don't use it. I used to be real religous about changing oil until I started using this Mobil 1. If you use their filter, it seems to last even longer. BTW, those KN air filters are pretty nice too; they last much longer and are easy to clean.
When using a weapon for defense, it STAYS clean. I'd wrap my rifle up to keep sand, and worse, dust out. Still, I'd clean it everytime I had down time. We'd wrap the M2's and Mk19's up real good --NOTHING sucks like cleaning a good greased up one of those after a drive through moon dust. My carry pistol, a Glock, it stays clean too, although due to the design it can go much longer than just about anything else, so my target pistols tend to get real quick cleanings. Not having to oil them up a very much really helps. When you depend on it, carrying dirty gear is kind of stupid --it'll never work better dirty than it does clean if it is worth using.
Finally, carrying an M4 or MG during daily downpours or constant moon dust presents a challenge. This one unit of POG's, they did the dumb army stuff. They'd clean their rifles DRY after an exercise, they may have learned to use pressure washers with hot water watching us but didn't do the other stuff afterwards (bad to do, but it works great every once in awhile with a MG). They may have been using shaving cream too, that is another trick (who figured that out and how I don't want to know). The rifles looked real good, real clean and would pass a white glove test. No joke. Then when they actually went to a range to shoot 'em (they shot just enough to qualify, once or twice a year) these BRAND NEW M4's started having parts failures. In at least one, the bolt carrier broke in half. The part had basically rusted from the inside out over time. Never seen anything like it, but it wasn't the only one in that unit to have problems, the barrels were just trashed but brand new. So sometimes over cleaning is bad, especially if you remove ALL of the oil on the metal, cleaning with water, yet keep it dry enough not to rust.
Seen one guy try and armor-all his pro mask for an inspection, but that is another story. Yep, seen it all.