I agree, modern EFI measures air density, so just being warm shouldn’t make any difference. One thing it might help is better fuel vaporization, but I don’t know how big a factor that would be.
Most exhaust systems that are slightly more free flowing than factory exhausts are also loud. This loudness causes many people to go easy on the gas so they don’t get pulled over and that in turn may cause them to get a couple of extra miles per gallon.
If you have a gasoline engine in the truck, the big airflow restriction at normal speeds is not the air filter or exhaust system, it’s the nearly closed throttle. Whether it’s the air filter and throttle, or the throttle alone that restricts the air to a certain power level doesn’t much matter to the engine.
With diesel engines, there is no throttle so any airflow restriction causes extra pumping losses which cause the engine to do more work, lowering fuel efficiency. You may see a slight fuel efficiency gain in a diesel engine with a free flowing exhaust or air cleaner.
[b]K&Ns work great. And, there is no need to oil the thing after it’s washed out and dried. Been using them for 2 decades with great results on light trucks - Mazda B 4 cyl, Ford Ranger 6 cyl, sports cars - Supra and Jag, ordinary passenger cars, Cressida, Camry, Olds Aleros, Chrysler Sebring, Lexus 330, with no problems whatsoever. Unless greatly improved throttle response is a problem.[b/]