Keeping flex in old vinyl

[Grimly]

Chuck's query about re-dyeing his seat covers got me thinking...
Anyone know whether there are any polishes or cleaning agents to avoid on vinyl seating?
Reason I ask, is I'm fairly sure my current seat cover should have lasted a bit longer than it has, but it's not OEM anyway, so is of undeterminate quality. I only recall it wasn't all that cheap and wasn't rubbish when new.

[chuck hahn]

Mine are sun faded mostly. So too much outside storage or sun slamming them all day is a recipie for deminished life and asthetics in my opinion. Try to park at work that will limit the direct sunlight as much as possible.

[Grimly]

The cover was replaced about 20 years ago - had about eight years of use, then parked up for ten. It started deteriorating within a month of being pressed into service again, with the vinyl going hard and cracking up into the usual lumps, where it was under most stress, at the front half of the seat.
I'd occasionally cleaned it with silicone polish but not too often because that just made it slippy, and I'm wondering if the silicones could have affected longevity. Otoh, a decade of use, in total, isn't too bad I suppose, but it just seemed to start falling to bits fairly quickly.