I'm actually doing both... heated and cold filtering. I heat the oil overnight to hopefully drop out some of the water and to get the oil warm enough to flow through five grades of bag filters.

After this filtered oil cools, I pump it through a 5 micron cartridge filter on the way to the truck.

By the way, when the oil is still warm, it flows very well through the bags. This week I pumped some through wihtout heating. Average temps in my shop are probably 90 degrees. Even late night the interior temps are over 90. Just this much cooling reduced the flow rate significantly. The bags filled up very quickly.

The thick tallow isn't a particulate contamination. Could we use a higher micron filtration than 5 microns to catch this 'sludge'? Would a 25 or 50 micron filter catch the tallow?

Otherwise, it seems to be up to the heat exchanger in the tank to heat it up sufficiently to flow and burn in the engine.

Todd