We have a 1996 Grand Banks 46 with 350 hp Cat 3116's turbos. We use the Cat recommended oil; SAEO "Special Application Engine Oil" which is specific to the 3116 and 3126 marine and truck engines and is available from Cat distributors and at most Cat service centers. It is a straight 30 weight. We just bought a couple of cases for $22.00/gal in Florida, which is no more expensive than any good quality diesel engine oil. We have had zero engine problems beyond normal maintenance and now have logged 3,500 hours.
The 3116's that were problematical were manufactured in France and are generically known as "soft block" engines. Most failed in the first 500 hours. Call Cat with the serial numbers of the engines on the boat you want and they can tell you if they are soft block from France. Good luck.