Finasteride has been shown not to have any effect on sebum production, almost certainly because it doesn't affect the type of 5a-reductase in sebaceous glands (which is type 1). There was even a study a few years ago that found no effect on acne from a specific type 1 inhibitor, which really really makes it hard to believe that finasteride could have any useful effect.
I was given propecia from the Doctor last year. I took it at one pill a day as directed and within a few weeks my skin was literally dripping with oil. My back broke out heavily in acne and my face got terrible cystic acne. I had always ha d very clear skin up until this point. I stopped the drug and let it get out of my system for a couple months and tried again at 1/4 pill every other day. Within about a month the symptoms returned. No acne product could touch the stuff. I was also using Nizoral every day, sometimes twice a day to try and combat this problem but to no avail.