Thanks for the information! I checked the manual and It list's everything from 20-50, 10-30, 20-20, 20-40, 20-30 from different manufactures. Delo makes a 20-40. Am I over thinking this?
Yes.
The manual should also be showing temperature ranges in which to use each of those oil weights. Pick the oil that best fits the temperature range when the engine will be used.
For say, 20W50, I assume there is a W and the W matters, every manual I have seen would show that a 20 weight will have a lower air operating temperature while the 50 would have a higher air operating temperature. Given the high temperatures in many engine rooms I would think a 20w40 or 20w50 oil would be a good choice.
Just make sure the oil meets or exceeds the manual's API Service Classification, the two letters in the "donut" on the oil container. Read this link, but make sure you read the part of the document for diesels, not spark engines,
oilspecifications.org - Lubricant standards and specifications.
For most diesels, you want to use a diesel rated oil not a gas rated oil.
I use a variety of oil brands. Which ever oil meets the manual specification and for the best price. I have used Shell 15W40, Shell 5W40, Delo 15W40, JD 15W40, and JD 0W40 on my diesels. My JD riding lawn mower and DR Mower are using a synthetic, I think it is 0W30, that might be Penzoil.
The oil lab I used for used oil analysis did a test a year or so ago with oils that were 30-50 years old that they bought off Ebay. They ran the variety of ancient oil through some of the their cars with no problem. Just pick an oil that meets the engine specification.
Just changed my oil and I forgot to mail off the sample.... Note to self...
Later,
Dan