Every car is different. The car that you are talking about might have low compression or other issues( clogged filters, clogged strainer, etc). My 84 starts right up. If it is cold outside, like 30-40 degrees, it cranks a for 2-3 seconds and you might have to throttle it a bit , but it starts, and within 2 minutes it runs perfect. My wifes 86 SDL ( OM603 motor) starts even easier, and runs even better, but they are different motors, different injection pumps, bigger battery, etc...
I regularly use the Lubromoly diesel inj cleaner. I buy it by the case. I change the plastic filter, and use lubromoly every oil change or sooner, and clean the Lovecraft filter every 1500-2000 miles. No issues.
I understand that the best setup is a two tank system, and I have the valves and lines to make it a two tank but since it works fine, why bother. The only thing that used to bother me was the supposed coking of the piston rings, but since I am not getting any blowby, or loss of power, or gelling, I am not worried about it. I adjusted my valves 2K miles ago, head looked great. I changed my oil pan about 4-5K miles ago because it had a big dent, and there was no gelling in the pan either.
My oil level is always perfect, car barely uses any oil between oil changes. About 1/2 quart between oil changes, with a slight oil leak, on the big pan.
So i am just driving it and that is it. The 84 300D has faded paint, red interior, it slow, but it runs, and its free, so that is all it matters.
I also have 2 spare motors apart ( OM617) so I am ready for a rebuild if something happens.
The price of an engine rebuild on the OM617 is $500 ( valve job, rings, gaskets, timing chain, tensioner) or so its cheaper for me to keep driving on veggie oil, and build one of the spare motors than spend money on diesel or gas.