TLDR: My lifetime knowledge about motor oil in one easy to read effort-post.On the Duramax forum I used to belong to the general consensus was that oil is oil up to a point.
I worked for STP in the late '80's and heard this straight from the engineer that did their vehicle testing at the time, with the disclaimer, "you didn't hear this from me." If I recall, they got a dozen identical Grand Ams and ran them on treadmills for 100k miles. They got scheduled oil/filter changes during the test, then got torn down at the end for measurements.
Long and short of it was, only minor variances in the additive packages across brands. The sole benefit of synthetics is heat tolerance. They'll run about 150º hotter before turning into sludge. Shutting down a hot turbo engine is where that really matters. Change it on time, keep it full, and don't run it hot and if turbocharged do a cooldown idle is the way.
A few years ago, I seen an article on the topic of diesel oils on Turbo Diesel Register (homepage, I did not search for the specific article). Within that was a chart where they got an analysis of all the leading brands. Rotella had the best, followed closely by Delvac, with Delo in third at a distance. I did not look much at the specialty brands, as my interest was with semi trucks and the big 3 brands are what is available most places. I see Valvoline mentioned, I don't remember where it placed. Experience with gas engines it's been a quality product. Just not usually offered in truck stops and big truck parts houses like the other 3. I would presume it's good as Rotella or Delvac, but nearly never see it where I shop.
In the KW I drove (C15 CAT) I did a 15k mile interval. Delo would noticeably begin breaking down after 10k. It was more watery on the dipstick and oil pressure would drop 10-15% by oil change time. Delvac and Rotella would not do that. They remained normal until drained at 15k.
My Ram pickup doesn't get the miles, so gets changed about every 12-18 months, so I just pay the toll for T-6 Rotella. If I was running hotshot and changing it every 4-6 weeks, I wouldn't worry at all over using T-4, provided I remained a stickler about a 3 minute idle cooldown before shutting the engine off. With T-6 that's not as important, but I still do it. The big price jump happens going from T-4 to T-5. Stepping up to T-6 is a much smaller increase. Not enough savings to justify not buying T-6 over T-5 IMO.
The irony is my new Macks have a 50k service interval, and any brand has held up like a champ. They have the MP-8HE which is basically a Volvo D-13 painted red. What Volvo/Mack done different to extend intervals is: EGR comes from after the DPF to keep soot out of the engine, and they have factory bypass filtration.