John,
Any dressing will make the seat VERY slippery (ask me how I know

). To clean, I would just use some warmish water lightly put onto a rag and get the dirt off. If yu get a lot of dust in the creases, use a wet Q-tip to get in there.
As long as you keep it from really prolonged periods in the sun, it should not be a problem. If you are going to store it over the winter, then by all means use a little vinyl dressing, by the time Spring comes around, it will no longer be that slippery anyway.
HTH
Gersh