alt Hacker NewsOn older heavy equipment of low value and high difficulty servicing (think like a forklift or skid steer) it's not uncommon to replace the coolant with oil to mitigate a head gasket issue and simply drain some oil and add to the coolant on some semblance of a schedule.
One of my favorite tractors was the old Oil Pull's which were designed for oil in the radiator. (they were a gas engine, but designed to run on "tractor fuel" which is closer to diesel than gasoline - in order to work the engine had to be very hot)