I use 15-40W Rotella T in my Chevy Duramax and change it every 5k miles. Is Syn worth the extra cost? I do plan on keeping this truck for a long time. Any better mileage with the Syn? I am getting 18 mpg everyday driving and 10 towing the fiver.

I am thinking about this but using mobil 1.
chevman

In a word,...YES,use synthetic. I've been using Mobil-1 in my Motorcycle, lawn mowers, pickup, & wife's car.
You will likely see a decrease in engine oil temp, quieter engine operation, & increase in fuel mileage.
I specifically noticed less engine noise in my motorcycle, mileage went from 49 mpg to 52 mpg riding the same route at identical speeds & conditions.

That is a decision only you can make for your situation. Read: this irv2 thread for days worth of research condensed into that thread.

Although I'm a believer in synthetics because of their extended performance capabilities, whether or not it makes sense to use them really depends on the vehicle's operating profile. I use synthetic oil in my 2003 Acura 3.2TL-S that's my daily commuter car as well as in my 2007 BMW K1200GT motorcycle and will switch Madame Boomer's 2008 Toyota Highlander over to synthetics as soon as her 3 free oil and filter changes are over - at about 10K miles. However, I run good ole Chevron Delo 400 in my truck because it accumulates very few miles (it only has 38K miles right now) and is used almost exclusively for towing the 5th wheel. Because of this, I change oil in the truck every 6 months regardless of mileage to get rid of condensation, combustion acids, etc. On average, these oil changes take place around 3K miles, so synthetics don't make a lot of financial sense in my truck's application, especially since Delo 400 has a history of exceptional performance in diesel applications.

Rusty

I changed to synthetic on the wifes Suzuki and noticed an immediate 1.5MPG increase. Same primary driver, same rout to work and back, this was from one week to the next. This is not the first time I have seen this in one of my vehicles. On my D/A I did not notice any improvements. But it was not a daily driver and used only to tow my fiver, so those numbers could be skewed some.

I used synthetic oil for quite a while-I had the oil tested and was running high iron content.I have switch back to Rotella,tests are normal now.Cost with standard oil about 50 dollars with filter-synthetic is about 120 dollars.4 gallons to change oil,5000-6000 miles change interval.
Rich