Dan, I've had no problem running a mix of 20% used oil/ diesel right in the regular tank. Had a couple of nights down below freezing so far and started up without any major incident. I ran as much as 35% (rough estimate) without problem this summer but don't think I'll try that when it's cold. The stuff in my barrel stays liquid at least down to the low 30's (barrel is in my garage, but I've gone collecting in freezing weather and the oil I got had viscosity somewhere between 30W engine oil and 90W gear oil. Still liquid.

I've got a boat tank down on the workbench. Hope to finish that, maybe get a Greasel style setup installed this weekend. Once you have the heated tank, outside temperature doesn't matter. As soon as the engine's warm (and therefore the veg oil tank and fuel line are as well) you can switch over from diesel. Just have to be sure to switch back before shutting down to flush the veg oil out of the IP with diesel again.

I ran my Golf through a whole winter on the Greasel setup with no problems at all.

F