I would recommend buying one of those "marine gear oil transfer" pumps for filling the trans after draining it. Remove the bottom plug to drain the old oil. Replace the plug. Remove the top plug (both plugs are on pass. side of trans) and pump the new fluid in with the transfer pump. You can get the at the discount stores for under $10.
I don't have an opinion on the Red Line or Royal Purple. Couldn't hurt, though.