Here's some stuff:
SAE 30 was the recommended viscosity. Otherwise known as 30 weight.
Apparently, at the time "DAY" was a service rating (I haven't heard of).
The service rating thing has changed since '72. You'll never find oil with that rating today.
Talk to a Volvo marine shop that services marine stuff (not a new engine sales guy).
Oil has changed, ratings have changed. Probably any SAE 30 (30 weight) with a rating for diesels (starts with "C") is as good as the original spec.
Multi-viscosity oil (SAE 15w-40) may be a better choice.
A Volvo Marine Service shop will know the answer. The NAPA guy? Maybe.
The quote of 8 hrs. is unreasonable. The guy doesn't want that work.
Guys know that oil, fuel filter, oil filter changes on sailboats can really suck. It can be very hard to get at stuff. Yes, it should take 2 hrs, or less, if everything is accessable.
Diesel fuel filters:
I'd advise anyone with a diesel aux to have the ability (knowledge, tools, filters) to change fuel filters, and bleed fuel lines. I'll explain this advice, if you ask.