I changed the engine oil of my Suzuki Swift VDI at 19,344kms (12020 miles)
The car didn't even run 100 feet (about 30m) before I checked the oil again, and it was very black in color and had poor viscosity.
All engines retain a small amount of residual oil even if you let the sump drain for awhile. If you do not change the oil filter then it will also be storing some oil in it. For diesel engines, particularly, it is advisable to change the oil filter each time you change the oil. Don’t be surprised if you change the lubricating oil in your diesel, run the engine for two minutes, and check the dipstick only to find that the new oil has turned pitch black; this is normal and not a reason to change the oil again immediately. It is simply the small amount of residual oil and the carbon soot that coats the oil galleries mixing into the new oil.
Your owner’s manual tells you the maximum interval you can wait between changes, but the more often you change the oil on any vehicle, the longer the vehicle will live and the healthier it will be. That goes double for diesels because extreme heat and pressure help to contaminate the lubricating oil more quickly. The cost of having a professional change the oil on a diesel engine can be greater than on a gas engine. This may be extra motivation for doing this relatively simple job yourself.