How cold does it get in Estonia? As it gets colder, the oil can solidify and then it won't pump any more, and you've definitely got a problem then.
I've driven an OM616 motor on old fryer oil in the northern US winter, probably as cold as 10-20 F so maybe -6 to -12C with no issues, but it was a 2 tank system, with a heated tank and lines for the fryer oil. Start the motor on diesel, switch to oil when warm and flush with diesel before shutting it down.
The first motor in it had wicked blowby that only got worse, the second one was OK.
A friend of mine drove a W201 190D with an OM602 motor on just straight fryer oil, and would switch back to diesel when it got 'too cold', I never did determine how cold that was. I did notice the little heat exchanger that heats fuel with hot coolant right out of the head on my OM602. That, of course does you no good till hot coolant is coming out of the motor though.
What fouls the rings or hone is usually a debate about WVO burning, and partial combustion from starting with cold WVO comes up at the top of the list.
Look up this guy who uses the name 'John Galt' on biodiesel and WVO forums.
http://www.make-biodiesel.org/Drying-Oil/cold-upflow-settling.html . What this page talks about is a system to separate the fats out of WVO to lower the pour point- the lowest temperature it still pours at.
He says he lives in the extra cold part of Canada and burns a diesel/vo mix all the time. I think he keeps his motor under an electric blanket in the winter.
Best of luck!