Richard, as far as I know there is NOT an additive which goes in engine oil or gearbox oil, and the additives which are available, like Silk50 etc can do more damage than good as they contain teflon which adheres to oilways and expands with heat and can block the smaller bearing oil feeds, leading to premature failure of bearings.
The only additional engine and gearbox oil cooling that I know of is the addition of a cooler, which is basically the same as your water radiator, but smaller and able to take higher pressure.
To acomplish both extra engine oil and gearbox oil cooling you would have to have 2 quite separate units fitted.
If your car has a turbo, then the additional oil cooler is probably fitted, for engine oil.