You'll be hard pressed to see any difference in synthetic. Think about how little a difference 1/2 mpg gallon is.

IMO its tough to prove that a gain that small (as reported by some in this thread) came from running synthetic.

By the way,Rotella 5w-40 isn't really "synthetic" Its just highly refined dino oil.

 

Synthetic enough to be able to carry the name. I can't afford (refuse) to pay for amsoil price or mobil 1.

 

I thought in the summer months, you had to run 15w40? can I run 5 40 all year round?

 

I thought the same until we bought our 05 Passat and it uses 5w-40 year round, and that engine works a lot harder than mine. I thought why can't I do the same with my truck. I noticed on the +90F days with the A/C on I was running 200F or just slightly higher, now in the same conditions it doesn't go over 180F. I would not have believed it myself, and now this winter was a bear and -15F it starts like it was 50F, never plugged the truck in and never failed to start. With 15w-40 and my other dodges I would not have started in that weather unless plugged in.

 

I've also heard alot of guys running the 5W-40 year round. If it is a good synthetic I think you would be fine.

 

even with the latest price increase in october- i get three gallons of amsoil delivered to my home for 69.00 bucks. this gets me 20,000 miles/16 months before my next oil change!!! i come out ahead!!!!!!!

 

Dodge & Cummins state that you can use 5w40 year round as long as the oil you use is Synthetic. BTW you can not use 5w40 synthetic blend oil in the summer. I use Amsoil 5w30 HDD year round and my UOA reports show its doing its job with out any problems. Now if you haul real heavy (gross weight above 18K) and are not using group 4 or 5 synthetic oil then I would use 15w40 in the summer.

JMHO

 

valvoline 5w-40 isnt to expensive, around 21 a gallon.

 

Dblr "BTW you can not use 5w40 synthetic blend oil in the summer."
This is unequivically a false statement. Blends work fine in the summer.

 

As far as I knew rotella 5w-40 is a syn. just a different group than others such as amsoil and you dont want to go as far between changes as the more expensive syns but you can go farther than on dino oil. And I wouldnt run 5w-30 my book says the engine should take dino 15w-40 or syn 5w-40