I'm not sure which compressor I have, I can check this evening when I'm closer to the car. You think this has some bearing on the decision?

Whoops didn't see your other comments:

That is an interesting idea about using a spare battery to heat the coolant. Thought about doing that to heat up the battery as well with a little heating pad. Has anyone tried this sort of thing?

The glowplugs are a year old and pencil type. I reamed the holes this summer. I'm replacing the valve stem seals this weekend to see if that helps at all. I'm not sure it's reasonable to expect starting much below 15F though. Do you?

The oil is Rotella synthetic 5W-40.