I've had my mountain bike for three summers now and I love it.  I bought it for the ability to ride on different terrains and build endurance.  Since my primary goal is weight loss and cardiovascular health, I've found that I ride mostly on paved surfaces during typical rides.  I do often ride on bike trails for long endurance rides.  These trails consistently have crushed limestone surfaces, as well as pavement.

I've considered purchasing road tires due to my extensive presence on pavement, but how well do road tires perform on crushed limestone and/or tightly packed gravel?  Do road tires sink a little on crushed limestone, causing lots of friction?

I don't want to spend the money on the conversion just to find the answer on this question.

I will be riding on paved & crushed limestone/gravel during the same ride.