How often should you do an oil change on a diesel engine?

As often as the manufacturer recommends.

Some people try extended drain intervals with Synthetic.

I wouldnt do that unless you have a bypass oil filter too because the extra soot will wear out cam lobes faster than normal.

The frequency of oil changes for a diesel engine varies, and it's always best to consult your vehicle's manual. Regarding using 5W-40 in a Duramax, it's crucial to follow manufacturer recommendations. I found some useful insights on engine oil choices on the Moto Deets Automotive blog. They have a detailed guide on Duramax engine oil, and it helped me make an informed decision for my vehicle. For specific and reliable information, checking out such automotive blogs can be really beneficial.