How dare they dis diesel oil!! Its the greatest thing ever!! How could the people who made, tested and raced the engine possible know anything about it??? I run diesel oil and its the best stuff, got a uoa to prove it plus I read it on the internet ya know.
Sorry I couldn't help myself.
I suspect you will not get a good answer on here. Maybe send Yamaha an e-mail and have them explain the reason they state this?