I'll give you my opinion, which will not even get you a cup of coffee.
As far as engine oil is concerned, I doubt that synthetic oil provides much advantage over dino oil under normal operating conditions. However. under extreme conditions of heat and stress, syns apparently will out perform standard oil. I put the syn in to give myself peace of mind that I have given my engines (all of my vehicles and generators) the maximum protection that I can provide. Hopefully I will never test the limits of the syn oils.
When I had a gas powered coach, I really appreciated the fact that I had the syns in the engine and tranny when I was crawling up the long grades in the Rockies. My GM powered Workhorse worked hard for 15 to 20 minutes at 4000+ rpms while going up some of the grades in the mountains. I felt confident that the syns were paying for themselves at that point. Additionally, I had no problem with extending the oil change interval to 6k to 7k miles. This allowed me to do an entire trip out west and return home without having to do an oil change.
With a factory recommended oil change interval of 15k miles on my diesel, I'll change the oil once per year or 15k miles. In reality I put on about 12k miles per year and change the oil. I do this just prior to our annual 2 month summer trip so that I have "fresh" oil in the engine when we leave. The rest of the year is spent in good old flat Florida where the engine is almost asleep as we go down the road, so it's under a low stress situation.
I use syns in all other applications on the coach also- trans, front bearings, rear axle, and chassis grease.
'05 Dutch Star 4015, '16 Rubicon OlllllO, & HD Ultra ElectraGlide, NKK14278L