My car will be due soon for a rear axle fluid change. The bottom of the axle near the back plate is black and oily. I presume it's a small leak as there's t no oil on the garage floor. I asked a mechanic some time ago and was assured it's Ok as long as it doesn't drip.

I intend to change the fluid to synthetic. Would the leak get worse with synthetic just like changing the engine oil from dino to synthetic?

Thanks a lot.