I run two stroke oil, TC-W3 non synthetic has lower ash content then diesel fuel itself from what I research, 1:200 is a good guideline but I am closer to 1:250. I also use plenty of fuel conditioners, PS white bottle, whatever else I can find on sale (usually contains Xylene, petroleum distillates, tri eythlene 1,2,4 something or other-avoid alcohol). I have owned it for 60k miles (08 R320 CDI), it has 120k total. I run the best fuel I can, usually 47-48 cetane. I change the oil at 6k mile intervals. I use an oil flush product every couple of oil changes. Why so often, I have done some research and the DPF regen cycle adds a ton of fuel through the engine, I don't like the idea of diluted oil, plus there is thought that the oil cooler seals are failing from excessive blow by, and longer intervals lead to more blow by as the oil degrades. There were also some talk of engine failures from oil sludge. I have noticed the oil starts to go down around 5-6k miles hence I change it rather then top up. I notice the same thing on our tdi VW, it will start to use oil once it gets to a certain mileage.
I have recently removed the EGR for inspection, it was clean without any buildup of carbon, nothing like the photos online!
I don't use ATF, it swells rubber parts I would not want to find out what is rubber in the 642 fuel system.
Yes to diesel purge, however it is impossible to run it off the can, there is an intank fuel pump and pressure sensors and such feeding the high pressure fuel pump. Prefill the new fuel filter with it, dump the rest in an empty fuel tank?
Change the air filters often, you will understand why once you have changed them a couple times 40k is way too long.
Now for biodiesel, B5 is the max according to warranty concerns. My understanding is the DPF regens and results in oil dilution, the byproducts of biodiesel are really bad for the engine internals and end up in there each cycle. Higher concentrations just make the problem worse. This is the only reason I have found that makes any sense, and usually what the dealers are blaming oil sludge on.
All North American 642 engines have a DPF.
Talk with any injection specialist and they will comment that fuel is dry, adding something to help will maintain your injectors and pumps. Parts are not serviceable on these engines, and at $4000 for a set of injectors, plus thousands more for pumps and sensors/rails you get the idea!