I've had pretty good luck with using a full can of Seafoam in fuel for a "new to me" genset that has sat for a while. Seems to help dissolve & burn any gummy fuel residue in fuel system.
I know that knocking sound you are referring to. Usually that means one cylinder is slow to fire, once it does the engine smooths out. How long are you pre-heating with Glow Plugs/Intake Heaters active? I hold preheat for at least ~30 seconds, more (Full 60 seconds) when it's really cold.
Have you verified that Glow Plugs & Intake Heaters are working?
BTW, I use BioBore (longterm storage) & Optilube XPD (added lubrication factor) and that combo has worked well for me in my gensets.
For reference
Here is a study cited in this BITOG Forum that was done a while ago by a highly regarded testing firm "SouthWest Research Insitute" (SWRI). Optilube XPD came in 2nd. This study is a bit dated, however Optilube XPD seems to have stood the test of time.