Keeping oil temperatures under control is a key component to the durability and longevity of any engine that sees regular heavy duty use. Overheated oil is thinner and breaks down faster than oil kept within its specified operating temperature range. Vehicles that utilize the 7.3L PSD are commonly used as a work horse that often operate in heavy duty applications. Ahead of their time in 1994, Navistar International with their T444E was designed to have an external fluid to fluid oil cooler, therefore, the Ford 7.3L Power Stroke did too which is definitely a feature that helps improve the durability and may be a contributing factor for its proven long life span.
The factory oil cooler design uses coolant as the heat transfer medium for the engine oil to regulate its temperature instead of air because fluid to fluid is more efficient at heat transfer than air to fluid. The OEM oil cooler on the 7.3L PSD is about as good as efficiency as you can get with little to no design improvements outside of the addition of more coolant passages in later years.
Mounted below the drivers side exhaust manifold the log style oil cooler design has proven to be a long lived component with a very low failure rate other than O rings. The external location makes accessing the cooler for O ring replacement or service a fairly easy task.
It is important to note that proper service and maintenance of coolant and engine oil helps prevent blockages and clogs in the oil cooler passages which ensure it can last a long time. But as the O rings age they may begin to leak or drip, especially when cold. When the O rings start to fail when the engine starts and is cold the O rings will seep or leak oil but then as the engine warms up and the O rings expand the leak stops. It is a good idea any time you are under the vehicle such as for oil changes that you check the oil cooler for signs of leakage. At the first signs of failing O rings you should plan to perform a reseal of your oil cooler by removing it and replacing the O Rings because the problem is only going to get worse, fix it while it is still a seepage or a drip. You will need to drain coolant and oil to perform this job.