Anyway the oil got full of sludge because of overcooling. It doesn't run hot enough to evaporate moisture and condensation.
They ran them with 160° thermostats or no thermostat at all, because glycol hadn't been invented yet.
They ran straight water in the summer and alcohol in the winter. The alcohol would evaporate above 160°.
So they overcooled and sludge built up in the oil.
I put a 190° thermostat in it.
Diesel trucks also overcool when idling. So I'd assume they'd also build up sludge when idling overnight in cold weather. But I've never heard of it happening.
You probably won't get any sludge in an oil sample because it stays in the engine when you drop the oil.