Antifreeze

This may be a dumb question but can antifreeze settle out? I have an old nat gas powered pumping unit that has been sitting in the barn for the last 4 years. Before I parked it I changed the oil and antifreeze and then ran it for a few, maybe 10, minutes. I pulled it out a couple weeks ago and the antifreeze in the rad was slushy close to solid actually. I opened the drain on the block and it was fine. Put it in the heated shop and let it thaw out and took a closer look at it today. Antifreeze from the top of the radiator tested good to -8 celsius and the first stuff out of the block was good to -40. I mixed it all back up and it tested good to -29 which is about what it would have been when I put it in. How is that possible?

That initial 10-minute run time was not enough to thoroughly mix it.  You should have run it up to temperature so the thermostat opened and kept it there for at least 10-minutes to circulate and mix the entire system.

I always premix 50/50 in a five gallon bucket coolant to distilled water ratio with ethylene glycol. Have never seen it separate in the past. Typically I use "Prestone" for automotive use and "Fleet Guard" in low silicate formulation for industrial and truck engines.