Many tire places will fix leaks for free. Almost all tire places beg you to not use the goo except in an emergancy. They have to clean it all off when you change the tire eventually. So go get it taken care of.
I had a tire that was dropping slowly for a few weeks recently. One night at 1 AM on the highway after 50 miles of driving, I felt the handling go weird and realized I was getting a flat. Luckily I was prepared to change the tire (spare properly inflated, flashlight, etc), and I had a safe place to do it. The tire in question had a nail in it, and probably had it the entire time but never got hot enough for the hole around the nail to stretch and lose more air.
Every other time I've had a leak, it always turned out to be a nail or something similar. The lesson is that tires rarely just "get a small leak." They usually get punctured by something that stays in the hole and keeps the air from all rushing out. But eventually its going to get worse and be a safety issue, especially if you drive long distances at high speed.