Oil leaks very slowly out of 86 Bronco. Will running thicker oil fix it?

I have a 86 bronco that leaks oil very slowly, but the oil falls onto the clutch making it jerk. I've ran some of that "Stop oil leak" oil additive and it did actually help, but hasn't completely stopped it. My uncle suggested running thicker oil in it to make it harder to leak out. What weight oil should I use? I live in Mid Missouri and it gets pretty cold in the winter so nothing that will be too thick to start on a cold day.