Different brands flow different when cold.This was a cold flow test that someone done on various synthetic oils.
This video isn't an accurate description. It's not in a controlled environment.
looks like Mobil 1 is something to consider.
I looked, they do have a 10w-40 Diesel Synthetic.
Why cant you switch back and forth Synthetic/non synthetic?
You can switch back and forth between them, but why would you switch from a superior product to an inferior one?
I believe in using multi-grade oil for diesel engines. Because we have 2 VW Jetta diesels. The same oil is used in John Deere diesel is synthetic 5w40 Castrol for diesel.
Even if your owner's manual says SAE 30 or any straight weight viscosity, you are okay to use a Mult-grade viscosity oil, as long as your numbers match!
For example, SAE 30 and 10W-30
The first part of the multi-grade is the viscosity that the oil flows at during temperatures less than operating temperature. typically under 180-212 degrees Fahrenheit. So even in the summer time, when it's 110 degrees Fahrenheit out, your oil is not at operating temperature.
So it's better to use multi-grade for pumpability. Which provides better protection, since you're not starving parts of the engine that need lubrication.
There are a lot of misconceptions in oil. I hope I was able to answer some of them.