You should really always use leather with leather and synthetic with synthetic, otherwise you can damage and shorten the lives of both saddle and leathers.
They'll rub the leather. If you want leather webbers Bates do some thiugh they aren't cheap, the best price I found was www.4dobbin.co.uk (or may be .com, I can't remember) & I've always found the service excellent.
__________________ I was so young and full of pride / And you were wild and strong; / I never knew how weak I was Oh you speak to me in riddles and / You speak to me in rhymes