I use Delo in my boat (5.7 Indmar Motor -GAS) so i will use Delo in my rig as well.

 

I see no reason why you can't go 7500 or longer on your oil if you’re between schedules A & B. It's the steady diet of stop and go for and hour or more per day, a lot of runs under 10 miles and the engine does not get up to operating temps and idle excessively that hurt the oil and thus the need for shorter drain intervals.

Heck I've run 5-7K on my cars & trucks starting way back in the late 70's and I have never had an oil related problem with any engine. I would worry more about cheap oil filters then the oil it's self that is unless you use extra cheap no name oil. If you use questionable filters then change them out every 5,000 miles and add a new quart of oil.

The oil they make now days is 10-20 times better then what they had back in the 1980. IMHO the extra 2,500 miles on each oil change over the life of the engine will save you a lot of $$ and I don't think it will be any worse for wear.

It's the places like Jiffy Lube, Oil Can Henrys etc. who came up with the change your oil every 3,000 miles ideal and not the auto or oil manufactures. They are only in it for the money and nothing else. At one time it was found out that Jiffy Lube estimated that if they could get us to change our oil sooner then the 3,000 they could make a few million dollars more per year in profit. This is one more reason why at one time they would give you a few dollars off if it were with in so many miles of the mileage listed on the reminder sticker on your windshield. To them it's all about money.

BTW Delo 400 is a good oil to use in your CTD, use to use it in my old 350 Cummins.