I would say if don't change if you have been fine on reg delo 400. No need to spend the extra money if you dont have to.
Ive been running the delo 400 for the past 5 yrs without issue. I tried the synthetic this winter because I had the romps for the first time but turned out to be a failed injector. I also ran it to help with sub zero starts. Im back to 15w40 though. I have better batteries now too, so I will see how the 15w40 does this next winter.
Use the if its not broke, don't fix it route. Oil technology in general has come a long way, not just synthetic.