Went the full synth AmSoil route with my C90T, sounded like the engine was beating itself to death. Not what I expected. Went to the Rotella T6, not any better.
Have run the Rotella diesel non-synth in it since then, purrs like a kitten.
This is not to say that synthetic is bad; but don't be afraid to go back to dinosaur oil if your engine is too noisy for your liking.