I owned an 83 GMC Jimmy with a 6.2 NA diesel and ran it to nearly 200k miles. I owned a 6.5 Yukon TD to 260k miles. Both engines saw various brands of true diesel oils in straight 30, then 10w-30, 15w-40 and 5w-40 oils. Some were non synthetic, others like 5W-40 were synthetic. I used Mobil Delvac 1300, then Mobil Delvac 1 synthetic as well as Caterpillar straight 30, and a lot of Shell (both 15w-40 and 5w-40). The switching of brands was due to availability and I moved to synthetic as it came on the market (and ran it in extreme cold).
Both engines were still original when I sold the trucks (except injectors and pumps), they both still started great at very cold temperatures (-10F) and I was approaching 3000 miles before a quart of oil was needed. I had oil analysis done from time to time and was told I could run to at least 5000 miles but never did go that long. I'm pretty anal about staying on top of maintenance and that carried over to the fuel injection, glow plugs, & starting system. The 6.5 injector pump driver was relocated for the last 100k and that helped. The takeaway I learned is all of the oils work well. All of them will allow long engine life. Spend more for the 5w-40 or even 5w-30 diesel synthetic for easier cold temp starting. I'd be surprised to find fault with any major brand as long as you change oil and filter when you should and use a decent filter.
Both engines were still original when I sold the trucks (except injectors and pumps), they both still started great at very cold temperatures (-10F) and I was approaching 3000 miles before a quart of oil was needed. I had oil analysis done from time to time and was told I could run to at least 5000 miles but never did go that long. I'm pretty anal about staying on top of maintenance and that carried over to the fuel injection, glow plugs, & starting system. The 6.5 injector pump driver was relocated for the last 100k and that helped. The takeaway I learned is all of the oils work well. All of them will allow long engine life. Spend more for the 5w-40 or even 5w-30 diesel synthetic for easier cold temp starting. I'd be surprised to find fault with any major brand as long as you change oil and filter when you should and use a decent filter.