Joe,
What size engines do you have now? Are they modified? If so how much power are they making? How much oil capacity do you have?
Oil capacity is a huge factor in controling oil temp which in return is a big factor on engine longevity, cooler temps (200 to 230) make for happy engine parts.
Marine engines run long periods at constant rpm much like endurance racing engines. Endurance racing engines have large capacities of oil. Nascar uses a 5 gallon tank. This allows the oil to be cycled slower through the engine then compared to the time it would take 1 1/2 gallons. This volume of oil takes longer to heat up since it has more volume.
We have seen as much as a 30 degree drop in oil temp in the boat by just changing oil pans. . .going from an 8qt to a 12 to 14 qt pan.