IIRC, a 5W-40 oil such as Mobil 1 Turbo Diesel Truck (TDT) or Shell Rotella T6 5W-40 oil flows down to -65F, so no issue in Maine during the winter. A 5W-40 oil is what's required for summer use so just use the 5W-40 oil year round.
Don't worry about the old oil appearing totally
black during an oil change. Newbie diesel owners often freak out when they look at the oil for the first time. It's 100% normal and nothing to worry about. The new oil will appear totally
black within a minute of running following an oil change, often before the engine is started again. Diesels push soot into the oil and TDIs are no exception. The newer CR TDIs take longer to blacken the oil because they run so incredibly clean but they too will blacken the oil. The soot is too small for the oil filter to catch it and a diesel rated oil controls soot by keeping it safely suspended in the oil where it does no harm. A small amount of old oil remains in the engine during the oil change and this is enough to turn the new oil totally
black in no time at all. The oil appearing
black simply means the oil is doing its job and it SHOULD appear
black. The time to worry is if and when the oil appears anything other than totally
black.
Stick to a 10k mile change interval by the book. The 10k mile interval has been verified through laboratory oil analysis to be perfectly fine and well on the conservative side. Your 100+ miles/day driving regime is just like my 100+ miles/day driving and is easy on oil. And driving it like you stole it and for long periods (not short trips) are the best thing for it.
Good luck and
Welcome to TDIclub.