Oils and science
This subject came up around February, 2021, and numerous times before that. Winter provides fewer alternative uses of our time, so I went deep then and before on lubricant research, a minor obcession - family member headed up a major oil companies specialty chemicals division, which included things like oil for manned spacecraft gyroscopes, open cockpit world class racing cars, additives and base stock for motor oils, etc., plus my own lifetime obcession in keeping cars a long time (22 years on my Silverado, 260,000 miles, long but not really unusual), reading Tribology Research from time to time, etc. (Yes, I do have a life - wife, church, golf, yard, consulting, expert witness work, volunteering, etc, although my reading Tribology Research might cause one to think otherwise.....)
Here's my bottom line: Do your own research. Read the back of the oil jugs. Go to Shell and Exxon diesel oil web sites, and others if you like. Read trade industry sources on trucking engine lubrication research, search for marine diesel lubrication research, etc. I suspect that TF friends who are still in the Straight 30 or 40 Dino oil world will have to read the research to be convinced that tribology science and application has progressed in the last 60 years. In the late 1960s working summers in a gas station we still had customers asking for straight 30 long after multi-viscosity and API ratings (SAE - Society of Automotive Engineers earlier) came out. Some things never change.
Of particular interest and easy to find are Exxon's and Shell's product information on their web sites. What is notable is how they compare performance of full synthetic multigrade oils to blended dino/synthetic and dino multigrade oils for shearing at high temperatures, deposits, engine cleanliness, acidity, and much more. I reviewed Shell's product info this morning - amazing how much better performance is attained by Shell's T6 over their blended T5, and how much better the T5 is than dino T4. No opinion here - just a referral to the research based product claims by the manufacturers of two of the largest diesel oil companies in the world. Trust them over any opinion I might offer.
Here is the site for blended T5, for instance:
https://rotella.shell.com/en_us/products/synthetic-blend-oil/t5.html
Look at that, be sure to hit the "for more information" tab at the bottom for specific comparisons, and do the same for T6.
Lastly, look up your engine manual recommendation for oil grade. Track how that grade rolls up into more recent grades - what older grades are covered by current grades. American Petrolem Institute has that information on their site, and most oil manufacturers do, too. (I have a great Volvo engine and they sell Volvo brand oil, and their grades can be traced from decades ago up their current grade, I think it's VDS4.5, and it covers the grade recommended for my 2005 TAMD41P-A.)
So, the information is out their from tribology research groups, the oil manufacturers, the American Petrolem Institute, trucking fleet managers, and all sorts of other groups and sources if you take the time to look it up.
I wouldn't admit to this in a bar or with my golf buddies, but it's actually fun.......
My conclusion for my diesel: Shell or Exxon full synthetic for heavy duty diesel engines, 15-40 viscosity (from the engine manual), most advanced grade that rolls down through the original grade in the engine manual.
Happy Researching!