+1 for the synthetic.

I switched to Shell Rotella T 5W-40 full synthetic last year mid-winter. I live in Wisconsin also, and we had one of the longest, coldest winters in quite some time last year. I noticed an immediate difference in cold start situations. Obviously the truck still needs to be plugged in, but it still started on the first try every time. I bought mine at Wal-Mart for $19.99 a gallon. That's a pretty small price to pay for the extra insurance IMHO. The nature of the Power Stroke injection system causes the oil to be sheared down rather quickly. The cost of replacing an injector or worse makes the justification of a good quality synthetic oil a no brainer. The oil is probably the most important part of the PSD engine! And everyone here is well aware of the service reputation of the 6.0L PSD! So why not take good care of it!?