Opinions on which would be best for CK27 gear
The one that costs the least.
All I know is that I just did my 50 hr maintenance and the spirax quieted the hst substantially. I can't speak to the mobil, but I assume its specs are similar to shell and it's also a recommended replacement oil. I agree, buy what's cheaper and give it a shot.
Is your HST loud? I might have had a little noise with my CK30 HST, but the DK45 HST makes NO HST noise. You must be enjoying that cab, you're really racking up the hours fast!