The reason I usually don't recommend synthetic oils in big diesels (like class 6+ trucks)is largely due to some of the tings I covered in my previous post. First is the OCI length, it has to be REALLY long to justify the additional cost of synthetic oil. We have Series 60 Detroit's and C15 CATs in the fleet that are approaching 1 million miles on whatever oil I can get on sale (usually unocal or Delo). The only major repair that any of these engines had were the 60's when they were fairly new had a liner recall from Detroit.
I like synthetic oil in severe duty applications such as engines with HEUI fuel systems (not necessary but helpful) and high temperature situations. Big diesels use coolant to oil coolers which keep sump temps under control. I just think it's unnecessary and quite expensive for the average truck operator.
Don't get me wrong, synthetic oil is great stuff and if you want to use it go for it. Most synthetic oil is at least double the price of Rotella/Delo/Castrol to begin with. For long drains I think UOA's are a requirement, so that adds to the cost of the OCI. Granted I do UOA's regardless of OCI length, however the longer the OCI the more important UOA's become IMO.
I have used synthetic oil in some engines that the senior drivers had in their trucks. They got the nice trucks and the good drivers of the high seniority bunch got more HP. I had several Big Cams and 3406's that had been turned up. On a couple of these engines I started having liner and skirt scuffing issues when they started heavy haul routes in hot weather. With synthetic oil in these engines with good drivers operating them those engines still achieved a “normal” LTO.
I think that the return for the cost is better with a good dino/semi-syn like Delo or Schaeffers and moderately long OCI's are a better way to go. There are other choices obviously such as Delvac and Amsoil. They are good oils no doubt, but quite expensive. Amsoil showed excellent performance in a guys N14 with high fuel dilution, there is a UOA of this truck in the Diesel UOA section here.
The thing is, when you are looking at large fleets of trucks the numbers are the most important thing to look at. How long can I make the equipment last for a given dollar spent. For a fleet of 1 or two trucks there is also a margin of Human involved. On my own stuff I spent a little extra money on oils and other stuff to hopefully prolong the life of my stuff. At work I typically see a hard part fail long before any oil related failure/wear takes an engine out of service. Even in cases of worn out rings, most of the time it's due to either begin dusted (leak in the intake track) or idle time that washes down the cylinder.
At some point you have to decide if you want to play it by the numbers or take a reasonable look at the numbers and then add a little warm and fuzzy in just because. Sounds like you really want to run a synthetic, so go for it. You definitely are not going to hurt anything and your engine will probably live longer baring any mechanical failures.
