The oil debate is one without an answer. I agree with the 15w-40 and I use nothing but the cheapest Wal-mart diesel oil. I have run 450,000 on a VW turbo diesel using 15w-40 Wal-mart diesel oil with no problems. Doesn't even burn oil yet. I change my oil and filter every 3,000 miles or less without exception. All my tractors are the same with oil and filters every 100 hours on the meter or twice a year (spring and fall) regardless.
I really don't think it's about brands, it's about changing oil and filters every recommended duty cycle without fail. If the oil meets the API, it's going to do the job. One recommendation though, use Diesel oil with the proper rating, there is a big difference! Never use gasoline oil with the sunburst energy star in a diesel, you will regret it.