Synthetic 5w40 has been working for me very well in my service truck. I also add two bottles of Rev-X every oil change. An added expense, but well worth it. The first time I added Rev-X, the engine got better with every mile I drove.

 

I've always used Delo 15-40. This oil change I'm useing Delo synthetic 5-40.

 

rotella 15-40 is what ive used in almost everyone of my vehicles even gasers that's the only oil we'll put in are ranch equipment im considering give the t6 a try on my next oil change since winter is almost here I change my oil every 4k and fuel filters every 8k no problems yet

 

I use 15-40 mostly ROTELLA, but if DELO, or DEL-VAC is on sale, i'll buy those

 

I also run rotella t-6 synthetic 5w-40. My 2003 6.0l has 195,000 miles, since I started using it, it starts great in the winter time! I had it start one time at -27 without being plugged in, it started like there was nothing to it.