Many questions

[JimT]

I think Lonestar10 is on to something. Maybe start there for the exhaust leak.

Next, before digging too deep into the internals of the engine, check the EGR valve and turbo to make sure they are functioning correctly as well as doing a complete EGR tune-up. You said the truck starts up and initially runs smooth which makes me think what you're feeling could be EGR related.

You didn't mention, but I'd have to guess that you're using about 1 gallon of oil every 4-5k miles? My guess here is you're burning plenty of oil then the system is spending plenty of time regening, using lots of extra fuel and absolutely killing you fuel mileage, trying to keep the system from getting clogged up. If you haven't started having DPF issues yet, you're probably pretty close. Between the burnt oil, possible unburnt fuel and a possible exhaust leak that would also explain the white smoke. The residue in the DPF is smoldering at the start of a regen, before it gets up to a higher temp. My truck had a similar problem. I would start the truck up, get rolling and before I got out of the parking lot it was already working on an active regen. Early on this wasn't too bad, it would smoke until it got up to temp then the smoke would go away and everything was fine. Eventually though the smoke started lasting longer and longer and the regens would sometimes last all day. At that point I think I had a number of things that were contributing to the problem, so i can't say exactly what the cause was or how to fix it.

Beyond that you have your work cut out for you going through that truck and trying to undo the lifetime of abuse. You seem a bit new to working on this engine, perhaps you should consider taking it to Mr. Hagg in GA to get it in shape, then you can learn to maintain it properly from there. Trying to learn and diagnose and fix and operate all at the same time is not easy (i know, I've been doing that) so unless you're in a desperate situation maybe let someone else help, at least initially.

Absolutely try to eliminate idling.

Get that engine operating in it's full RPM range. Try not to operate below 1400rpm under any load except the first gear or two. (It will be tough with a 9 speed) That will be better on the engine and it will help get the truck moving better.