valve adjusting

Not true at all. The performance isn't the main gain here, fuel mileage is. Yes, it does need done. 100%. Don't listen to this guy.

 

Do them every 100k miles your engine will thank you

 

WOW, is there some serious bad info.
Lets look at this from a practical standpoint. We all know a cam is a solid piece of metal. It has no variable in it whatsoever. So is a rocker arm, a valve, and a valve seat. Nothing in this system varies, except speed with RPM.
That means every single cam rotation the valves open 100%, and slam closed steel valve onto steel valve seat 100% every time, hopefully without fail.
The steel on steel contact of valve to valve seat happens repeatedly every moment your motor runs! Can't stop it unless you turn off the key. Every single contact creates wear, it's small, but IT'S REAL! Do not ignore it. Valves and seats are tapered, as the valve and the valve seat wear, the valve slowly starts to sit higher and higher in the head. As this wear happens it slowly removes the clearances between the valves and the rockers. When they get too tight, then the rollers on the rocker ride the cam continuously because it has no clearance anymore due to the valve wear. This doesn't allow for proper oil flow into these very high stress components, leading to cam, rocker and valve failures! Very expensive indeed.
If you are diligent and adjust your overhead with frequent intervals and properly measure the valve protrusion, you will know when to change your head because the valves are worn enough that they are now riding on the edges where they are weak and will fail.

The manual for doing anything on a cummins is available for your engine at Quickserve.cummins.com sign up for an account. It's free with 5 or less ESN's and is a lifesaver. The step by step instructions for everything, right down to I framing your motor are on there! Fantastic resource.
Personally, I recommend an over head adjustment annually. Learn to do it yourself, it's not hard. I'm doing mine in a truckstop parking lot today!
Here is a great video of how to do an overhead adjust on an ISX by a driver for a driver...

 

At the end of the video, he says he uses "85-140 gear oil" instead of Lucas because of the additives. I thought gear oil harmed the bearings ! Go figure.

 

gear oil has additives that have been taken out of the regular oil due to epa. I would say at least every 200k. I think cummins recommends 500k but in my opinion that is too long.

 

Some gear oil will harm soft metals, particularly brass.
If you are going to do this, make sure you use GL-4 oil. Just because a bottle of GL-5 says it's backwards compatible, doesn't mean it's ok for an engine application.
He is an interesting and intelligent fellow. He's a former hydraulics field troubleshooting engineer, so he's forgotten more about oils than I ever intend to learn.

The most important additive, particularly for an ISX is zinc. And that is one that the EPA is pushing the oil companies to reduce. They haven't pushed that agenda into gear oils because they aren't changed as frequently. Certain brands of GL-4 gear oil have very high concentrations of zinc, and other additives that can actually enhance engine oil as opposed to diluting additive packages with some popular name brand "enhancers"
IMHO, even 200k miles is stretching overhead adjustments. I do mine annually, and remove the rocker cover regularly to inspect the DOHC's and other components. I check things over 3-4 times/year. Cummins' notoriously low oil pressure usually aids that premature wear. You'd be surprised what you can catch early and save yourself big money in doing so.
I have shimmed up the regulator in my oil pump and run between 40-45 psi oil pressure at operating temp and rpm.