My opinion?
Swap it for the V6 litre petrol version.
Seriously I dont think it matters how old fuel is in a modern vehicle. The tanks are basically sealed and unlike old vented fuel tanks of old to please the environ mentalists. So the lighter elements cant evaporate away over time to leave stale fuel. And so it really doesent matter. I do know that adding say 1% of 2 stroke oil to deisels makes them run quieter, less knock, and it lubricates the fuel pump, solenoids, injectors/pintel, valve guides and cylinder bores/rings better. And its something I always did on work vans years back. Any cheap low ash synthetic 2 stroke oil works great. Even cheap 2 stroke oil is better than straight diesel.
If you want to save a lot of money you can (and I have in the past) used cheap UK "heating oil" as its not dyed pink. You order 28 second oil. Instead of red 35 second diesel. This is slightly thinner, and so benefits even more from a drop of oil added to it. Say 2% two stroke oil. DONT use a red one! The advantage is that while not being "legal" its a lot cheaper! Like in summer it was 55p per litre. Currently its 74p. In summer demand is lower so the prce is cheap. Its delivered to your house by tanker. So you would need an oil tank. Or know a man with one. 28 second oil is also called kerosene in the UK.
https://www.boilerjuice.com/kerosene-prices/You can get 45 gallon oil drums delivered to your house for under £1 a litre too. But thats less economic...
In the US kerosene and parafin are basically the same thing. Or at least refered to as the same thing. They are not, at least in the UK. And they smell different too. Its hard to tell diesel/kero by smell.
But basically kerosene, 28 second oil, heating oil, kerosene, jetA, JP4, JP8, all work great in a deisel engine and are very similar. I wouldnt hesitate to use any of these. So does red diesel, but its 35 second oil (same as white diesel but may have more sulfur that doesent matter). And of course because its died red (so called marked) its also illegal like all of these alternatves here. And when the tank is dipped by the roadside its too obvious!
The others are normally the same colour as white diesel. So you cant easily tell...
So in answer to the first question, it doesent matter if you empty the tank or keep it topped up. But to me a full tank is like money in the bank. And thats better than NOT having money in the bank.. So keep it topped up! And keep a couple of 10 litre cans in the back or at home. You never know when theres going to be a shortage or a strike or whatever. Right now I am having two 45 gallon oil drums of 28 second/kerosene delivered for my diesel heater! If I had a diesel van then that would be rather to easy!
Premium diesel is basically a way of extracting More money. It has some chemicals added to keep the fuel system clean and stop organic growth, and water collecting. But I doubt you could ever detect any difference in use. It was originally intended for to be used in huge diesel tanks used in houshold heating systems or on farms where 1500 gallon or bigger tanks are common and where fuel sits for years to decades. I wouldnt bother with premium. I would add a splash of 2 stroke oil. And I do use a 0w 40 oil, (mobil1) in place of the 5w 20 reommended ENGINE oil. Likewise I will be swapping to a 5w 50 oil (synthetic) this year. Because I value the engine rather than a tiny increase in mpg.