Old thread but interested in the oil take.
New to the group but ran cycles for a long time and will be getting back into it with a Ural cT if all works out in 1.5 years.
Also repaired and maintained diesel engines for 35 years.
My take was diesels did not benefit from synthetic but did like good diesel grade oil of course. Some of it is so close to synthetic now that its as good in the high temp area (or so close as to make no difference due to the crud build up. Newer engines have changed.
I did run the Delo 400 in the cycles for a while. Just good quality oil, AK you don't need the high temp capability.
However, I also ran in cold temps and got caught out in some under 30 degree mornings and the start was iffy.
The machines got shifted to synthetic to solve that and no issues. All air cooled in those days.
I have a diesel Passat that requires synthetic oil. It has Unit injector (cam driven nozzle and injection pump in one unit in the head) and to stuff that in an older design engine, they had to make narrow cams. It needed the best extreme pressure oil you can get, ergo synthetic.
I run it in all my vehicles due to low temp starting.
You can run it longer as it does not break down as fast. The Passat oil change is 10k regardless of service and that is in a diesel and its not as clean burning as the newer ones are.
Where you can run into issues is an old cruded up but stable engine that you put in good oil that the detergent release the crud and it plugs up oil passages and moves into bearings.
Conventional oil that are oriented to air cooled engines are fine, long term a diesel or synthetic would be better.
What I do depends on where the cT winds up living. Stateside in the N.W. it will get Delo 400 or synthetic after the break in (and that is up in the air in my mind so far, 1,000 to 2000 miles likely). If it lives in AK then synthetic.
I may change my mind, oil is a pretty subjective area as the thread mentions. Not alwyas a direct cause and affected though suspicion is oil if that is what you changed and then it went South.
2 x CB750K (one a true Japan Model)
1 x CB700 SC
1 x R80GS (ok to start with, learned to love it for what it was)
1 x CB450K