Try to avoid driving it anymore until you get the fluids and filters changed. Especially the oil filter and fuel filter as they may both be paper, and disintegrating as you drove it home.
Considering that you're going to sell the car there is no need to use a synthetic.
Unless the engine has known internal issues - knocking, ticking, etc noises, there is no need to use a 15w40.
I'd also be careful about using a high detergent oil, you don't want to loosen up any gunk at this point and create a problem that will just cost you money at the sale. Just use an average, name brand dino oil in a 10w40 and a very good filter to catch the bits of the old filter media that may have broken loose and are looking to clog an oil passageway in the engine.
I'd be concerned about safety equipment failing on an old car like that before I'd worry about belts and coolant hoses. It would be a shame for you or a potential buyer to wreck the car on a test drive before you get the cash for it.
How old are the tires ? What kind of tires are they ? Is there a date code on them ? Old tires love to pop on the highway in the summer. And you cannot reliably look at a tire and gage when it will let go. I'm just saying.
Same thing for the brakes. If you don't when the rubber brake hoses were replaced last, all three of them, then you're really taking a risk driving it. Think about the rear brake wheel cylinders, the steel brake lines, the master cylinder, etc. Just a minor failure will increase your braking distance when you least expect it.
Check the steering shaft coupling very carefully. That connects the steering wheel with the steering box. Same goes for the tie-rod ends, pitman arm, idler arm, center link, ball joints, etc.
Has the rear axle oil been checked yet ?
Be safe.
Keith
