Owners’ Group: Using Manufacturer’s recommended oil in 30 year old engine

A Yanmar GM user asked the following question, and it's a good one…
IIs there any advantage to running straight #30 diesel rated oil as compared to 15W40 in these older 1980\’s engines?

Thanks,
James (who sails a Bayfield 32C in Ontario, CANADA)

Is there any advantage to running straight #30 diesel rated oil as compared to 15W40 in these older 1980's engines?