My Suggestion: Formula 303
http://www.303products.com/I've used this stuff for years. You can't buy it in stores. It is used by the military to keep seals on jets like new and air tight. This stuff rocks. It soaks in and makes even the oldest vinyl, plastic, and tires look brand new. And, it doesn't wash off easily. Finally, it does no damage (unlike ArmorAll), and has 15 SPF for sun / fading protection.
Give me an old car with dry, brittle plastic and vinyl, and I'll have those parts looking new in an hour. And they will stay that way.
My only caution: Be careful about painted plastic (e.g., the fake metal trim painted on plastic in certain areas of the Pinto like the climate control). This stuff penetrates, and can make the silver trim fade. Save it for seats, headliner, vinyl tops, tire, rubber seals all around, even the engine rubber parts. I've used it for years on old cars, and have never had an issue.