I saw on Spritmonitor that you use 15w40 .
Have you ever thought of going to a lower weight oil ? 10w or 5w40 ?
Especially the latter can have a positive effect of your diesel consumption in winter - mostly in the worst phase right after starting the engine.
WS2 ...
I've still got most of the fine grain Tungstendisulfide that I got from Vekke.
He used it in his oil and transmission on the 2L Lupo.
It's not going to go into the GasDwarf any time soon - factory warranty - so if you want to try it, I'll happily send it over to you so it gets used.
I only put it in the engine oil of my V50 - @ 1g / L - but didn't see many effects that I could attribute to it.
It may have been running a it smoother.
OTOH, it didn't hurt the engine either as far as I could tell.