What separates oil brands is their additive packages and its resistance to shearing to a lower viscosity from use. I've run used oil analysis' on two brands of oil in my 7.3 liter Powerstroke Diesel: Motorcraft and DELO. Further, I've seen the UOA for Rotella. FWIW, here's my thoughts.
First off, for those of you who are not familiar with the older Powerstrokes, they use an injection system that is referred to Hydraulic Electric Unit Injection (HEUI). The injectors are electrically controlled and hydraulically accuated (older CAT engines are also HEUI). Powerstrokes have two oil pumps: A Low Pressure Oil Pump (LPOP) for circulating oil throughout the motor and a High Pressure Oil Pump (HPOP) that uses the oil as a hydraulic fluid to power the injectors. The HPOP varies the hydraulic pressure to the injectors depending on engine load; pressure at idle is about 500 PSI and pressure at WOT is about 3000 PSI.
Using the oil as hydraulic fluid is very tough on it and will cause it to shear to less than a 40 weight over time; the question is how much time. Motorcraft (the OEM brand) sheared much faster than DELO. It just barely hung onto a 40 weight rating after 3100 miles whereas DELO was comfortably within the 40 weight range after 4500 miles. DELO hardly shears at all at 3000 miles. The Rotella UOA I saw showed it shearing less than Motorcraft but more than DELO.
In terms of additive packages, DELO has one of the best in the business (Shaeffer's probably being best). The antiwear package DELO has is designed for increased engine life.
One other point: Switching brands frequently is not recommended. It usually takes a motor an oil change interval or two to get used to a new brand. It is quite common for trace wear metals (iron, copper, lead, etc) to spike for a while before settling down after changing brands.
The above all could be meaningless for the application we are interested in here. Is a superior antiwear additive package necessary in our hydraulic systems? If we change out the hydraulic oil once a year, how much does shear matter? I will probably do a UOA just out of curiousity on that last point.
Clean oil is the most important point of all.
Best,
Jim