I have almost no sheen on the water.
The fact that you have a sheen means you have a problem. That generation of engine, like our FL120s, will put a sheen of unburned fuel on the water at cold startup and for a few minutes after that as the combustion chamber temperatures are coming up to normal. If he sheen doesn't go away after a few minutes then your engine is not running at its optimum.
Our engine exhausts put a sheen on the water behind the boat at cold startup. Five minutes later, give or take, the sheen is gone and there is no more evidence of unburned fuel coming out the exhaust.
When we bought our GB we followed the advice of the folks we talked to in the marine diesel manufacturing industry (and the engine manual) and use single weight conventional oil, in our case Delo 400 30wt. We have been doing this for 14 years now. Here are the comparison figures for our engines from when we first bought the boat in 1998. Since buying the boat we have put some 2,500 hours on the engines. The numbers apply to each engine.
Coolant operating temperature in 1998--- 180 degrees
Coolant operating temperature in 2013--- 180 degrees
Engine start time in 1998--- immediate, as soon as engine cranks.
Engine start time in 2013--- immediate, as soon as engine cranks.
Oil change interval in 1998--- 100 to 150 hours.
Oil change interval in 2013--- 100 to 150 hours.
Lube oil consumption per engine between oil changes in 1998--- 1 to 1.5 quarts.
Lube oil consumption per engine between oil changes in 1998--- 1 to 1.5 quarts.
Duration of fuel sheen after startup in 1998--- approx five minutes.
Duration of fuel sheen after startup in 2013--- approx five minutes.
Engine oll leaks in 1998--- none.
Engine oil leaks in 2013--- One, from the fuel lift pump gasket on the starboard engine, very common FL120 leak, repaired last week.
NOTE: We had the stock raw water pumps on both engines changed due to the finite life of the Lehman drive coupler for the pump. These couplers are no longer available and according to Bob Smith (who designed it) they are not repairable. Well, they are, but according to Bob the repair never lasts long.
The pumps we installed have a much greater water flow than the stock pumps. This resulted in a significant lowering of the transmission temperatures (a Very Good Thing). However, this much greater flow did not affect the engine's coolant temperatures by a single degree. Why? Because the thermostats in our engines are working properly and they continued to keep the coolant temperature at the proper setting, same as they did with the smaller, stock pumps.
For this reason I am VERY skeptical that simply switching oil types would have any effect whatsoever on the coolant temperature of an engine unless the thermostat was defective, failing, or of the wrong type.