I would use a good filter with a magnet on it, but I wouldn't waste Amsoil for a crankcase flush. I'd use a 20w-50 petroleum oil and kerosene (or trans fluid, or diesel fuel), 2-3 times should make it better without splitting the case. I'd ride it lightly too. Bumps and leaning can only help but clean the sump out.
These motors are pretty dang sturdy.