I looked into the oil requirement for a new to us Amel with a Dessalator 160, CAT 277 pump, early last year.  The CAT manual recommends an ISO 68 hydraulic oil.  I believe this is a 30 wt. oil.  I would agree with Bill Kinney that a 40 wt. oil would would also work well.  The book recommends a change every 500 hours with the ISO 68 oil or 300 hours for if other oils are used.  The manual did not specify only an ISO oil, I believe SAE oils have detergents and other modifiers added.  You can reference the manual in the Files Section > Dessalator Technical Info > CAT Pump 277 Svc Manual, p6.

Good Maintenance

Mark Mueller

Brass Ring  A54