Being the optimist that I am, I'm going along with what Ron mentioned in his first paragraph...the "cyclic resistance".... which is easy to identify if it is the case. When your timing mark is showing top dead center, every degree move of the crank moves the pistons very little... all the pistons being at either the top, or the bottom. Rotate 90 degrees from there, and all the pistons are in the middle and travel their farthest distance per degree of crank movement. Another 90 degrees, you're at bottom dead center with two pistons up, two down....

If the rings are somewhat dry, this could be a cause of added resistance...though, not a whole lot. What are you using to turn the engine? Just your hands around the crank pulley? Or a 3 foot breaker bar? It will obviously be much more noticable if just using your bare hands....

And, if this is the case, it would be easiest to turn at top dead center, and bottom dead center... stiffest exactly in between. easy->hard->easy->hard per crank revolution. (not twice per firing order) Should be gradual in between.

As others have said, if it is easy to turn, then becomes stiff suddenly (or worse, you hear a clunk)...that could (would) be something much more significant.

I would first identify exactly where the stiff spots occur. If they are consistant with what I said above, try adding a little oil down the spark holes to lube the rings. See if that helps. And...if you are certain there are no noticable clunks or sudden stiffness, put the plugs in (just to get compression...don't hook 'em up), and turn it over with the started. Listen carefully for ANY inconsistancy in how it turns by the starter.

Or, maybe it would be best to leave the plugs out and try the starter with a very weak battery to keep it from spinning too fast... just a thought.

As with others...keep us posted!
-Matt