Heres the story. Truck is a 2011 F350 long bed crew cab, SRW bought brand new by my former boss in Sept '11. He sadly passed away in March at the age of 25, leaving behind a wife and two infant sons. He owned a marine diesel company, truck was constantly running the roads until his passing, it wasnt uncommon to have a complete 6-71 Detroit, or some other ridiculous weight, in the bed. Truck overheated multiple times on a trip from georgia back to louisiana pulling 18k pounds, which led to him deleting it. I mean, it was ran like a dog before, but once deleted at 40k, (5 inch straight pipe and H&S Mini Maxx) he would leave the mini maxx on the hot setting and run the [censored] out of it. never any trouble, usually got 15W40 Rotella at 9-10k (had to remind him all the time lol) His father in law bought it in March, and it only has 73k miles now. He wants me to do the oil changes and fuel filters,but my question is, he doesnt tow anything( a 16 ft Carolina Skiff) and has a 10 mile round trip to work, and doesnt drive hard AT ALL. So should I go with 15W40, or Rotella T5 10W30, at 6-7k ocis?