Break in periods should be variable speed for the most part because while accelerating, there is more downward pressure applied to the rings from combustion, and while decelerating (still in gear, engine braking) the vacuum applied helps to hold the rings against the cylinder walls to allow them to seat under pressure (quicker, more accurate break in time).

Those are the only two realities that I know of to the "old ways" of breaking in an engine... it doesn't really matter what oil you use, synthetic or otherwise, except there is still a preference for high-zinc oils, which are normally dino-based.

No offense to anyone intended, but "break it in the way you intend to drive it" isn't really a valid remark anymore. It used to be the general consensus after rebuilding an engine, that if you intended to hot-rod the engine, you should rev the piss out of it, and let the engine slow the vehicle down as much as possible, but on the same hand, if you intended to use it for a fuel miser, you should take it really easy on the motor.

Modern technology and procedure has now shown that no matter what your intentions are, you should probably just semi-hotrod the motor anyway, at least for the first 500 miles or so. That doesn't mean redline it every shift and let it brake back down to a stop, but don't "Baby" it either.

It's like sanding a piece of metal... light strokes make it uniform, but brash sanding gets the job done faster. The key is to start out brash, then allow the parts to mesh together until the result is just light sanding. Proper oil changes and maintenance will virtually eliminate the end sanding effect, making the engine run properly for a long time to come.

Dyno "breakin" is a myth, in most cases. They're not broken in on the factory assembly line - they're tested for output and obvious problems. Think, though. On that dyno, the engine runs for a few mins at nearly 100% load... and it's never been run previous to that. You're not going to blow the engine up if you're smart about revving it.