The Mobil 1 5W40 is likely to be virtually immune to thermal breakdown.
So is Mobil 1 10W30.
The usual selling point of 10W versus 5W is that 10W oils are usually more resistant to thermal breakdown and shearing out of grade.
The only Mobil 1 I've ever heard accused of shearing out of grade is Mobil 1 0W40, and even that problem is only a rumor.
Basically, if you've already committed to a quality synthetic, there is no reason to choose 10W over 5W, although it wouldn't hurt in warm enough weather.
If your sister lives in PA, you'll probably want to have her running Mobil 1 0WXX or 5WXX for the winter and then 0WXX, 5WXX or 10WXX for the summer for cold starting reasons.
I believe that the standard BITOG answer for "it's a Durango with a V8 and it's been running 70K on dino" is as follows:
1. Order Auto-RX from site sponsor Auto-Rx .
2. Follow the directions for Auto-Rx to the letter. That'll get that V8 nice and clean.
3. Use a quality synthetic oil after the Auto-Rx cleaning. The oil selection is where things get dicey, but for availability (and it being my sister) I'd go with Mobil 1 5W30 or 10W30. Other members would be more inclined to mention Amsoil's various offerrings including series 3000 0W30.
4. Change oil once a year, preferably in the spring, right after it gets warm enough to get under her car.
--- Since it's your sister's car, I wouldn't personally do once a year. I'd be more inclined to do twice a year, which gives you 5,000 mile intervals. That way if it blows up, it won't affect your family ties.
5. Have the oil analyzed once by any site sponsor.
--- Terry Dyson would probably provide the best solution for this, but at $40 a pop, you're getting the works and paying for it. His package includes getting almost everything you can test oil for tested. Half of what you pay for with Terry is his expert commentary and explanation of what you see. He's probably saved a few million dollars worth of equipment in his career with his interpretations.
--- The cheapest single-shot analysis is Butler's Cat Labs at $12 a pop. They appear to provide a good service, and if you're too broke to spring for Terry, or know what you're looking for, they're an option to bear in mind.
--- Schaeffer's offers 6 analyses for $60. That's great if need 6 analyses. If you do it, buy it through site sponsor Specialized Lubricants and mention BITOG. Their analysis appears to be sound, but they seem to offer very little in the way of interpretation.
6. Post the used oil analysis on BITOG for everyone's viewing pleasure.
Disclaimers: I've never used Auto-RX, but all indications are that it's a good product. I have used Terry, and was very satisfied. I have used Schaeffer's, and I was satisfied, at least at the price. I have never used Butler, but I probably will in the future.
http://www.auto-rx.com/
http://www.bobistheoilguy.com/terry.html
http://www.butler-machinery.com/services/product_support_oil.asp
http://specializedlubricants.net/
