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SYM or SEM (I've seen it here both ways) has a good reputation for interior refinishing. I'm going to try it myself as soon as I get some stuff to refinish for my 77.
Never heard of anything called "SYM". But SEM vinyl dye is good stuff. I prefer to have mine mixed up to the required interior color code, rather than to buy something close in a rattle can. And, the sprayed liquid dye has the proper sheen for new interior parts; the rattle can stuff is a bit dull, IMO.
You can get it at any professional auto body paint supply store. If you don't have spray equipment, buy a Pre-Val bottle sprayer (very inexpensive) to spray your dye. It works really well with the thin, liquid dye.
The SEM works great for most of the interior, although I've never been sold on the concept of dying the seats, especially in a car that might be used regularly, with the top down, and/or in hot weather. I've seen it come off, but that could always have been due to improper preparation and/or application.
I think seat covers are the one interior part that need to be replaced rather than painted.
I used it on my 65 corvair on the door panels. So far it has help up to 115 degree temps. Time will tell how long it will last. I've only had it on for about two months.
Does the seat take a worse beating than your steering wheel? My [leather-wrapped] wheel was re-colored with SEM interior dye 4 years ago. Hot, sweaty hands, summer heat through the front glass, etc., etc......NO loss of color; NO chipping; NO peeling. And SEM ColorCoat is intended to be used on plastics, rubber and leather. If the seats covers are cleaned WELL (detergent, complete rinse, dry; final wipe with lacquer thinner if on leather) there should be no problem with it holding up for many years.