Hi Sparky,
After spending a whole lot time crossing checking the specifications of required by the manufacturer, I found that what West Marine carries, meets all the criteria. I think it costs about $18 a bottle.
Check for the source of your problem because this is not suppose to leak. In my leak, I found that the O-ring in the drain plug (the middle plug in the middle) was bad. The drip was small but consistent. The result was a pooling of liquid on the floor under the wheel. You spot these leaks by shining a flashlight at an angle across the gear housing. Later, I found a small but consistent leak in the front seal that is right behind the wheel and Andrew at the factory took care of that (Again, what service!).
For those of you who have not thought about this issue, open the plug at the top of the helm steer gear and take a look inside. You should see a fluid with a flashlight.