Thanks for the advice all. George is rather right about most points. It is indeed an aluminum clam cleat (Ronson I think), and the mainsheet line does not like to stay put in it. The mainsheet itself certainly has enough purchase in it...the main reason we keep the sheet on the winch is because it stays put much better on the self-tailer than it does in the lousy clam cleat! To say that I ALWAYS need a dedicated winch for the mainsheet is certainly going overboard. I need to be able to occasionaly free the cabintop winches for traveler adjustments and furling the mainsail. I'm really leaning towards the camcleat, but I really don't want to have to drill and tap that mounting plate to install it. Grrr....
Trying a different size line for the mainsheet may be an option, though I think my winches are bigger than 30's. 32's I think.