I see the shelves where I have been buying my Rotella 15W-40 is now inundated with Rotella Synthetic 5W-40. I like the idea of synthetic oil but I question the wisdom of putting that weight oil in my hard working klinker. The back of the container says that it is approved for Ford, Dodge and GM diesels, but it also says to check the owner's manual, which clearly states 15W-40. My truck never sees sub freezing temperatures, but I'm finding this oil on the shelves here in Mesa, Arizona where it never freezes. Should I use it?

see this post...

http://www.dieselplace.com/forum/showthread.php?t=190025

i had a similar question... everyone seems to ok 5w40, so thats what i run....

I have been using it since new, almost 80k. I change my oil every 10k and have yet to have a problem. We get everything from 10 degrres all the way to 100 here and have never had a problem with it.

DMAXBOB

No problem, the key is it is still a 40 weight oil..same protection.

The "synthetic" dark blue bottle 5W-40 is a group II based oil, IIRC. It's a very good product for the money. It's lower rating is great for people like me who see sub-zero temps in the winter occasionally. You can use it without reservation.

However, if you're staying around the southwest all the time, you have no need to spend the extra money on that stuff. You'll be fine by saving money and getting any conventional 15w-40; choose whatever brand you can find on sale at Wally World or any local auto parts retail outlet. My in-laws live out in Fountain Hills, and I know that I've seen Delo, Rotella, Delvac, Tection Extra, and other quality brand names on the shelves out there in 15w-40 when I last visited.

What temp would be a good break point to switch to the 5w40?

That's all I've been using for 50K now. No problems.

Again, as previously indicated, the Shella Rotella T "synthetic" is really not a true synthetic, as in Delvac 1 or Amsoil. Thus it does not have the wonderful low pour qualities of a group IV PAO synthetic. Same with the ultra high temperature performance, not even close..
So, ye gets what ye pays for... Great price with Rotella T 'synthetic' but not true 'synthetic' performance..
And yes, the 5W is the "cold" flow rating. It is a true 40W at normal engine operating temperatures..
George Morrison, STLE CLS
(and yes, it is a constant source of irritation for me with oil companies marketing "synthetic" labeled products, all totally legal, yet not with total integrity)

I woudlnt worry about running a good 5 40