Water in the oil

[Grant Bowker]

If you are ever likely to see freezing temperatures, I suggest anti freeze.
It doesn't need to be anything fancy, no need for exotic "aluminum safe" or "multi-metal galvanic corosion prevention" as it's all pretty much cast iron and brass that got along just fine on lowtech versions. An ethylene glycol based antifreeze giving lowered freeze point and some anti-corrosion should be enough. But I would stay away from the old-time concoctions (wood alcohol, salt, honey etc.)
A side benefit is that antifeeze/coolant usually also raises the boiling point.