The 03 has a stronger EGR cooler design (welded tubes vs thin brazed plates). Late 04 and up EGR coolers have a history of excessive failures. An EGR cooler from BulletProofDiesel fixes this issue for good.
Whether you call it weaker or not, I don't know. It is a fact though that there is a relatively high failure rate of 03 and 04 HPOPs. The 05 and up HPOP's are pretty reliable, but the original discharge fitting was prone to failure. Once the discharge fitting was upgraded, it was solid.
Regarding the oil rail - The 03's and early 04's did not have dummy plugs. They also had a different design for standpipes. They had fewer leaks than the later model years (late 04 on). The design for the dummy plugs and standpipes were later upgraded and have proven to be very reliable.
The original gold coolant is not very "durable" it can develop into a thick gel very easily. It can also drop out some of the hardness and develop scale. All this can plug the oil cooler and this plugging can cause failure. In general the OEM oil cooler is pretty reliable with the proper coolant. Problem is, most used 6.0's don't seem to have the CAT-1 rated ELC coolant. You need to be watching the oil temperatures and the coolant temperatures on a scan tool to determine if the oil cooler is beginning to plug up.
If, after the engine is FULLY warmed up, the oil temp is more than 15 degrees higher than the coolant temp ..... then you have a plugging oil cooler.
A scan tool is critical IMO to owning a 6.0L. Downloading ForScan Light to a smartphone is easy and cheap. You will also need an ELM327 OBDII adapter (WiFi for iOS and BlueTooth for Android). I like the reliability of the BAFX brand of adapters. Total cost is around $40. ForScan is a VERY GOOD code reader also. Some people like the ScanGaugeII because DieselTech Ron had videos on using it. It is a good monitor, but it is very weak on code reading. The Torque Pro app is like the ForScan app, but it has a few parameters that aren't always correct (EBP being one) and it is not quite as good at pulling all the codes as ForScan is. The Edge CTS line of monitors is pretty good, but expensive. They also have issues with accurately pulling a few PIDs (EBP and IAT).
I would never buy a used 6.0L without having a scan tool!
FICMs seem to fail fairly often - mostly due to low system voltages (weak batteries and weak alternator). Keep the electrical system healthy. The scan tool helps with this also. Once a person buys a used 6.0L, sending the FICM to FICMrepair.com for some reliabiliy upgrades is a good idea.
Lastly, these 6.0L trucks are all old enough to be in the 200k mile range. If the fuel pump hasn't failed by then, there is a likely chance that it will soon. Sometimes they fail almost immediately and sometimes they fail slowly. As they fail, fuel pressure drops. Low fuel pressure will ruin the injectors. Eight injectors is a BIG EXPENSE (apprx $300 each)!! Also for injector health, use the correct oil for the temperature in your area, and synthetic oil helps. Injector "stiction" is far too common and it develops with poor oil maintenance and the wrong choice in oil.
Injector life of 200-250k miles is fairly reasonable for a well cared for engine. Most used 6.0L trucks for sale are right around the age where injector replacement becomes VERY likely. I assume that is why MANY people sell their trucks at 175k to 250k miles.
Use ONLY the OEM oil and fuel (two of them) filters. Also VERY important - make sure the oil filter cap isn't a taller aftermarket version.
Every 6.0L owner needs an add-on fuel pressure sensor and gauge.
