You can add a lighter flywheel if your stock flywheel is dynamically balanced as an individual component. If the engine relies on a counterweighted flywheel to provide balancing the replacement flywheel needs to be counterweighted as well.

For an example, my S5 13b RX7 uses an 8 lb Guru balanced flywheel. To be able to use it on my engine, I need to bolt it on the Auto transmission counterweighted flexplate mounting flange.

As a side note if you add the 8lbs from the flywheel to the 8 lbs from the counterweighted flange the savings versus the stock flywheel are only 5 lbs, not the 13 lbs you would assume.

Eric