Shipboard synthetics used to be a no-no due to the fact that they broke down when exposed to water. This is no longer the case and according to my local Mobil rep, they now separate from water better than non-synthetics.
5 million dollar engine I am guessing is probably a big Sulzer type, 2 stroke with an exhaust valve instead of an exhaust port, with a crosshead system. Depending on the exact configuration, you COULD use a synthetic but with the consumption involved in your cylinder lube system your costs are going to go up some due to the difference in costs of the oil. Honestly though, I really dont know how much you would gain by doing this. As long as you are having samples lab tested regularly and keeping your purifiers/clarifiers in good shape, regular oil will give you the protection you need in your engine.
The biggest thing you need to worry about is fuel dilution. You can find this by using a flow stick once a month to check the viscosity of your oil.