I bought this truck with 280,000 miles on it. Only thing done to the engine I know of was maybe some IP intake manifold work. The only reason I say this is there is orange silicone where the turbo outlet connects to the intake manifold(where the egr valve sits on a S).
I have been doing upgrades on this truck to make it even more reliable.
You can see those in my sig. We are currently doing the cooling upgrade. While the timing chain cover was off, I had my brother-in-law(certified diesel mechanic) check the chain and gears. He says they look almost brand new and there there is no slack or carbon build-up. So what I am wondering-Did I luck out and get a great S engine? The timing chain cover, valve covers, have never been off till now. This truck was used to run newspapers from Twin Falls, Idaho to Sunvalley, Idaho. For an S engine(which some say S stands for slow) this truck hauls. Sorry for the ramble. Just wondering if I got an exception engine.

Is the engine certification label still on the driver side valve cover? If not, and no evidence of one in place, most likely has had engine replaced. If still there, you've got a good'un (or someone has worked on it and done a good job, you can hardly tell when a good tech has had an engine apart after a few months of dirt).

I can attest that a S can be made to run quite fast, means mostly converting to F though for max output, what you if chain is goot, next area of interest is the main webbing, since you are down, pull the pan and check the main webs, you very well could have got a good one or more likely one that wasn't abused before you bought it.