I replaced my gear oil, as I was having the same problem.  I can't remember the weight, but you want the heaviest, thickest possible. I got mine from a tractor supply store.  I also have a removable pin that will keep the winch handle from moving.  I keep the winch handle in mine all the time. 

I called Boston gear, and this is what they recommended.  Not very helpful otherwise.  Replacements are very expensive. 

Another thing I did was remove the starboard side of the keel box, so that I can see/feel the wheel as I lower and raise the keel.  Eliminates the need for counting.  Worst of all if you get excited, and keep cranking on the handle after the keel is all the way down, it can come off the wheel or break the wheel altogether.

Bob