Extended means more hours or miles than stated in the owners' manual.
Those book numbers are conservative because soot production/accumulation varies a LOT with engine condition and use patterns. But oil and filters are cheap so why chance it? By the way really large Diesel engines with really large oil sumps and have special filter systems to remove soot so they can use the oil until the lubricating properties and additives start to go bad.
I live in North Carolina in the US where we rarely have extreme low temperatures and I don't use the tractor if it's that cold. So I use the same oil (usually Mobile 1 Turbo Diesel) year around and change every 100 hours, regardless of season. But if want to start and use your tractor at -40 degrees you want a really low viscosity oil (0W), a block heater, or (preferably) both.