Robert, your last paragraph is exactly what I have been doing...watching the voltage while I drive with all of the accessories on. I guess my beast must be running properly, because I get exactly the same voltage numbers that you describe. What are the chances of that happening?
About the oil, I have been using semi-synth 5W30 and that's why the engine will start down to -5F without using the block heater. I got this stuff from a local farm supply shop. In summer I use 15W40. Switching to the 5W30 made a substantial improvement in cold cranking speed. I wouldn't trust my car starting at -5F with the 15W40 stuff, no way! I think it would be reliable down to only 5F. Also, the oil pressure drops about 2psi with the thinner oil though.