i'm not sure on your model but on mine i just go threw the menu on my dash and just turn on the high idle if your truck does not have the option than i would assume you could take it to gm and a tech cold turn it on for you ether way your truck has it built in
i work in the oil field up north in Canada and i see every kind of diesel starting and idling at very cold temps and i don't think there is any gain to idling any higher than 1000-1200 but at the same note high idling in temps any warmer than 0 is a waist of fuel in my opinion plus high idles can be hard on a truck if it idles s up to soon when its really cold i wish our trucks would idle longer before revving up my truck with 0-40 and a oil pan heater at around -30c and colder i need to start the truck and watch my oil filter while it idels up because sometimes it will gush oil out of the seal on the oil filter and trans filter for a second or two so you can imagine the oil press its seeing for the first bit
Automatic Elevated Idle that comes factory on the LBZ and I think LLY trucks in nice, but only intended to help speed engine warm-up in cold weather.
High Idle, is a completely separate thing. It is a on/off switch controlled feature. It actually is providing a "PTO on" signal to the ECM that tells it to raise the idle speed and hold it at the preset levels regardless of the load on the engine. There are 3 built in speeds that it will hold the RPM at, controlled by the cruise control switch.
PTO on - 800 rpm
PTO on - set cruise - 1250 rpm
PTO on - resume cruise - 1700 rpm
It is very easy and cheap to add only the high idle feature. You only need some wire, a switch, and 1-2 ecm terminals. Total cost can easily be $4 depending on how nice the dealer you get the terminals from is. Simply install the switch, tap into key on power and run to the switch, and run the output to the ECM terminals required for your specific vehicle.
As for idling at low idle. There is a couple of reasons not to do it for long periods of time. Biggest reason is that low idle results in an incomplete burn of fuel. It doesn't burn all the fuel resulting in cylinder washdown, and accelerated cylinder wear. Newer engines are much more advanced and the problem isn't as bad, but still there. Older diesels would actually die if idled too long because the rings wouldn't seal enough to create combustion.
It also doesn't get oil spread on the cylinder walls like it needs, resulting in even more cylinder wear.
It only takes a couple hundred rpm raise to get a complete burn and complete oiling, for most diesels 800-900 rpm is enough. It is also not something that is a problem in 1-2 minutes of idling, but the longer it idles the more damage is done. I up the idle to 800 if it is going to sit for more than a minute.
Most class 8 trucks will bump the idle a little when sitting overnight. On many newer class 8 trucks it is automatic, and not for warming the engine up like ours. I programmed many mack trucks to automatically ramp up when idling, back when I was working at the mack dealer.
It is something to think about. Also the down side of using a simple on/off switch is that it will limit your speed if you forget to turn it off. I used a push button on, brake pedal off system on mine that also activated an ignition override. So when I push the button the idle ramps up, and I can take the key out of the ignition while it is running and lock the doors. This works great for gas stations. When I hit the brakes it turns it all off, if I forget to put the key back in, the truck dies.
Anyway check out the ignition override how to thread, it says a lot about high idle too.