When I was detailing used cars for Ayers Chevy up in Edmond about 30 years ago, we would dress the top with Turtle Wax brand Tire Wax. The stuff came in 5 gallon buckets with a pump on top to dispense. Although it did not say Turtle Wax, it was identical with the tire wax in the green bottle at the auto parts store. Chemistry for auto reconditioning has change so much since then, Its like comparing WW II with Iraq. One of the advertisers on this site A1Auto has a link for convertible tops and the top cleaner and conditioner they sell is probably the best stuff I have used in 30 years.
As a side note, most of the "protectorants" that are used for tires and under the hood are to strong and will cause breakdown of the material that todays' tops are made of.
mike (94 V-6 Ragtop)