Last time I just used standard Merc drive oil. I have tried Royal Purple and Redline Shockproof synthetics trying to find something that would hold up against the sheety combination of water and gas that usually ends up in there. Doesn't seem to make much difference.
The main thing is changing it on a regular schedule to get the nasty stuff out. I change mine every time I change engine oil @ 25 hrs.
Here's pic of mine with the drain screw installed. Makes life a lot easier.
Regards,
Steve

Filled up with new gas and had no problem for the 4 day weekend!