Insequent,
Most engine oils will be suitable for use in marine gear transmissions as they are not friction modified so clutches should lockup soundly and without excessive slip or shudder. Automatic Transmission Fluids (ATFs) could also be used and would provide much longer life than engine oils. ATFs are friction modified and tend to smooth out shifts by laying down a very fine layer of friction modifier chemistry on the clutch friction surfaces. Check with the gearbox manufacturer to see if they now recommend ATF formulations. If so, you'd be best off with an Allison approved TES-295 product since it would be stable for many many hours (perhaps years). I wrote the Allison TES-295 specification when I was there.
API CD and CF are quite old categories. API "C" means "Compression Ignition" so these API categories define diesel oil specifications. Passenger car motor oils (PCMOs), on the other hand, will carry an API "S" designation meaning "Spark Ignition". The latest PCMO API Category is "SN". Whereas the latest API HDDO (Heavy Duty Diesel Oil) category is API CK-4. You'll still find plenty of API CJ-4 oils on the market and they will be quite good at providing viscosity and oxidation resistance along with all the other properties formulated into these products (seal compatibility, rust/corrision resistance, etc).
With regard to the "30W" viscosity, that I see being used here, per SAE, "W" means winter. So, it only applies to multi-grade products (SAE 5W-30, SAE 15W-40, etc). If it's a straight weight viscosity grade (30, 40, etc.) you should refer to it as "SAE 30 Grade". That is the proper use of these viscosity terms. You'll find engine oil viscosity specifications in the SAE J300 Standard (link to the SAE J300 table below).
Hope this helps !!! PS: I'm not here to write books; so, please keep it simple and look through my technical paper.
Links:
API CATEGORIES:
API | Oil Categories
SAE J300 STANDARD:
SAE J300 | Widman International