Recommended service intervals vary from manufacturer to manufacturer. My car requires oil changes every 5,000 miles (I change it every 3,000 miles). It explicitly states not to use synthetic oils.
I have been around cars all my life. Worked in many dealerships as well. One thing I know for sure is the automobile manufacturer has no interest in making your car last as long as it could.
Oxidation in today's oils are not as big a problem as it used to be. But, there is still the presence of fuel vapor, which over time, does break down the additives in the oil causing it to slowly lose the ability to suspend particulate matter. Many fuel additives expedite this process.
Diesel fuel is less of a problem than gasoline, which is one reason a diesel can go much further than a gas engine between oil changes.
Oh, mora, we have the same oils available here that you do. I use Pennzoil PZL Turbo, or Castrol GT in my engines. The PZL Turbo is difficult to find, but it really makes a difference in the smoothness and spinup of the engine.