Asia is not so easy with a modern diesel. Yes it can be done but there are risks. Russia is not a problem.
But in Iran the fuel is of low quality. In Uzbekistan there is little to no diesel sold, and you sometimes get it from a jerrycan or some big stationary tank, hoping that it is something good.
Having done a southern route and a northern route, I can say that it is definitely easier to find petrol than diesel, and that the diesel quality may be to low for Euro 5 and 6.
Whenever you get into an area where there is very little population and fuel is slightly harder to find, diesel is the first to have gone missing. Because the locals usually have some old petrol Hyundai, and the diesels are usually long range trucks.
Also, people liked diesel for what it was in the 80s and the 90s and beginning of the century. Simple reliable engines.
However, they have become so complicated that a petrol engine is more simple.