After X,XXX miles, the most oils WILL loose some amount of their viscosity over the life of the oil change interval. Do what RED said and use a 5W-30. It is better suited for colder climates.
The valvetrain noise you are experincing is normal to the 5.3L. Some people call it valve slap, some call it carbon build-up, etc. Whatever it is--it's just clatter of the valvetrain components when cold. Nothing to worry about, IMHO. You could try running Lucas Oil Fuel Injector Cleaner and Oil Stabilizer/Conditioner. Both smoothed out my cold-start up noises and gave better performance, MPG.
Is the Optima a Red Top? They tend to go bad versuses the Yellow Top. The Yellow Top ones are better.