I checked with some folks on another tractor board, and the concensus seems to be that the oil fills a bit more of the cylinder, bumping the compression up to aid starting.
Looking at the manual for that model of Lombardini, I also found that pulling out on the stop lever (vice turning it, which is what stops the engine) increases the stroke on the injector pump, squirting a bit more fuel into the cylinder which also helps.
It's 15º in my garage as of an hour ago. I put some oil in the reservoir, pulled the stop lever out and hit the starter and I don't think that ol' gal made two full revolutions before she fired off.
Not bad for a 40-ish year old engine. In fact, my John Deere should start that good!