What oil should I use?

[ChxrlieRxe]

Asking the Experts! Someone please settle this dispute---
I inherited Grandpa Charlie's Jeep - this is my first project. I've done a lot of cosmetic & cleaning work all over and now it is time to change the fluids and install the new battery and start her up to see what's what...she is a 1987 Laredo with 4.2L auto, 264,000 miles. It was running when Gramps passed in 2016, then garaged for 8 years and hauled to Colorado where it sat outside for 2 years until I claimed it. My Auntie told me she was driving it in Colorado when she hauled it there.., but it has not been started since. My brother is telling me to use

Shell Rotella T6 Full Synthetic 15W-40 Diesel Engine Oil​

why?? I do not have a diesel engine in the Jeep.... my owner's manual is telling me to use 10-30. I live in Phoenix- so we are at 1200ft above sea level and the temp never goes under 30, and usually hits 120 in the summer. I've attached a picture from the owner's manual- although, I know after 264,000 miles and almost 40 years the type of oil could change-- but not to diesel engine oil - right? RIGHT? Someone please tell me

 

[Gilaguy23]

My personal opinion is diesel oils are great in the right application, in summer??, OK. However I would not run 15/XX in your jeep in winter. If anything use one of the 5/40 syn diesel oils like shell T5 or T6, or whatever it is. If it were mine with that many miles id consider 10/40 high mileage oil or even a 20/50 only in summer. If you venture into cold country, White mountains in winter, your engine will not like cold starts with 15/40 or 20/50. High mile oil has more zinc, seal conditioners and detergents and Id guess at 250K miles you have some oil leaks. If you get bored check out PQIA for oil quality tests, additive packages etc.