I run 5w40. 30 weight is a little on the thin side, espeically on higher mileage motors as most of these cars are now. They can end up with dangerously low oil pressure when hot.
As for your oil light, i wouldnt think the 30weight oil is causing the light, most likely a clogged strainer or a worn out oil pump if its a particuarly high mileage engine. You'd really need a pressure guage to tell for sure though, and dont even consider driving it around until you've worked out why the light is on and fixed it, or you'll be replacing the engine.
