Plugs are where I'd start, and specifically #6. It's way at the back and most people have trouble getting to it, so I wouldn't be surprised if it's still the original plug.
2nd thing would be the distributor. I wouldn't just shotgun it, because there have been issues with getting the wrong dizzy for that specific engine. Open the cap and see if you can wiggle the rotor front/back and side/side. If you can, then the bearing needs to be replaced. BEST option is to get a new bearing and press it into place. Barring that, there's a marking on the housing that you need to match when you get a new dizzy. I've never done it, so I'm not sure what or where it is. On top of that, new rotor and cap are cheap and wouldn't hurt.
Last thing would be to replace the fuel filter. It's under the vehicle, near the center line on the last cross member before the rear axle.
As for the oil, nearly any 5W-30 will be fine. I would do a high mileage variant of your preference, since they have additives that help keep seals pliable and extra detergents to clean gunk.