Hope this helps
Change your outdrive oil and engine oils frequently. Can you believe that I have had owners tell me that they didn't even know that the lower unit is filled with oil? Well, it is. And not just any oil either. Use the specific oil that the manufactuer recommends.
Older and newer "R" and "Alpha" drives can use Mercury Marine's Premium Blend Lower Unit Oil but they now say it's better to use the green looking stuff called High Performance Blend. All the Bravo drives HAVE to use the High Performance Blend or they will experience clutch problems. The newer Generation 2 Alpha's also use the High Performance Blend.
It is from :
http://www.outdrives.com/merc/mdtips01.html