Outdoors should be fine. I have a collection of vintage 8mm movie cameras, and I shoot outside. Most of my cameras are manual exposure (where you set the aperture manually) but I also have a few cameras with an "electric eye" for auto exposure and they seem to expose fine.
Of course, the exposure also depends on the ISO of the film. Most 8mm movie films (like Ektachrome 100D) have an ISO of about 100 at the most. That's still pretty low, and you won't be able to shoot indoors unless you have really bright lighting. I've tried shooting indoors, even with the aperture wide open to f/2.8, and the film still came out too dark.
Just think about it this way...if you were taking still photos and you were using 100 ISO film indoors, you would normally need to do a long exposure. You'd have to leave the shutter open for several seconds. So a movie camera isn't going to do any better. You need to have enough light, whether it's a movie film or a still film.
You can email me if you have any questions. I shoot in 8mm and I might be able to help you.