Have you read up on leasing -- say did a goggle search on leasing cars? This is a decent explanation of all the terms involved
Car Lease Guide - Free Guide to Smart Car Leasing - LeaseGuide.com
All said and done, without knowing your situation, since you already have 8k saved you might be better off saving up a few more thousand for a down payment on a purchase. If you've got good credit you can easily get 4 % - 5 %. If your Z is around 30K out the door, with 11k down your monthly payment on a 5 year loan @ 4.5% would be about $354.
A simple bit of math might help you make up your mind:
Lease
320 x 39 months = 12480 + 8000 down = in for 20,480 after lease period
Buy
11k down, 30k price = 19000 loan @ 4.5% = 354 MO
354 x 39 mo = 13806 + 11k down = in for 24,806 after 39 months
At first it looks like you are in for less after 39 months with the lease, but you have only around 7434 left on your loan if you continue payments. Assume the Z is now worth 15k. That means you actually have $7566 worth of value in your car -- a little more actually if you were to sell the car outright and pay off the balance of the loan.
So
Lease after 39 months = in for 20,480
Buy after 39 months = in for 24,806 minus the value of the car that you own 7566 = 17,240
20,480 - 17240 = 3,240 less after 39 months
Of course this is based on a residual value of 15k after 39 months which may or may not be reasonable.