At 1500 miles, the gear shifts are just beginning to become smoother. I think the higher friction of the factory fill gear oil promotes the break in process as contact surfaces wear together and mate with each other. IMO changing over to a super slick synthetic blend too soon will inhibit this process. My theory is the proper time to change is after the gear surfaces have developed a good wear pattern; but before the synchros have had a chance to begin to wear and grind.