I couldn't pass this thread up and am a sure I'll overlap advice but here it goes:
Our diesels cruise at a steady rpm, temp, etc. so focus on single grade oils with lots of zinc because we don't have catalytic converters or EGR's to foul on our boats so we can use good stuff. Zinc and phospherous work together, in keeping your lifters rotating randomly so what is in this picture does not happen to you:
Sliding surface example of low additive package (You land rover drivers, get on this):
Single grade, vs multi-grade:
You also have an oil filter with a pressurized pump, so only use detergent oil, non detergent oils are for engines with accessible sumps like a farm stationary engine or oil field motor. Non detergent allows dirt to fall away from the gears, air compressors and lawn mowers etc. Briggs even recommends removing the side cover to deal with the dirt left over by non-detergent, this means that the dirt didn't circulate through the engine though... two schools.
When you think you've found your oil, there is probably a "used oil analysis" (UOA) someplace online for it. Diesel engines can use several API ratings, CH and younger are formulations to deal with the newer fuels of today:
Stay away from ILSAC everything, gas engines such as Atomics will enjoy SL oils or older, not SN or SM. C is compression ignition, S is spark ignition and progress alphabetically as the oils change for CAFE and National Association of Manufacturers decisions. There are only a few companies with lots of zinc and/or moly, phosphorous, calcium, etc, in no particular order;
Brad Penn, Joe Gibbs, Cen-Pen-Co, Swepco (had trouble with their 306 not pumping hydraulic lifters when 55degF though, their 502 is apparently the solution to their not great oil). I suggest staying away from synthetics as they were only designed for racing, emissions/CAFE standards.
Good oils of course are Delo 400 single and Multigrade, Delo 100. Diesel oils today on the shelf are typically MB 228.31 or MB 229.51 which are for Diesel Particulate Filters and low on everything we need. Delo 400 LE is one of the sneaky neighbor dogs. They're suppose to be backwards compatible but that only helps the store owner. You have to look hard and I recommend purchasing online;
I order Schaeffer's Moly-Bond SAE 20 in 5 gallon buckets for the Volvo-Penta MD2 and the same company supplies me 5-gallon buckets of Moly-Bond 15w-40 for my OM602A in a 190D Turbo. This is the most impressive oil to me and the most robust. If you don't start the engine but a few times a month, moly is your friend and Schaeffers uses about 200ppm of it. I'm comfortable saying that I've found the best oils for my applications, and not affraid to say that anything you're buying at Walmart/Autoparts chains are likely a compromise to your engine design.
Don't mix different makers of oils without manufactures approval, but DO find a proper fill and stick with it. Buying a different oil every time it goes on sale can gum up and create sludge in your engine with different additive and detergent packages.
Temperature, you'll notice that oils are rated in different temperature stages:
like 40degC and 100degC, without a temp gauge I can only suggest you let things warm up as the heat is required in the oil to work and clean. A multi-grade needs heat to expand the elastomer.
We're inundated with consumer type choices, consumers own things until they break, so we can't shop where they do. Unfortunately, you'll likely have to order online for everything mechanical in your future, your time and project is too valuable to walk into and out of a retail location unsatisfied.