Hi TTR, and apologies for the delay in responding.
I have no experience at all with Volvos, so I'm afraid I cannot add anything useful in regard to their seating.
However when it comes to "slippery seats" then its a different matter!
Rather than selecting jodhpurs, it may be better to wear a pair of riding breeches.....they come to just below the knee, but on folks of more compact stature are just about full length.
They are of course really comfortable to wear, having usually a stretchy component
(essential for riding, but supremely comfortable for normal wear), and best of all have either a leather "seat" area, either of real suede or alternatively a faux suede that seams to do the job just as well.
This leather seat area of the breeches grips well on leather seats (or saddles!) and is therefore a great safety aid and stops the slipping and sliding that other wise can be so troublesome
If the car seats have had some form of slippery polish or silicone applied then a wipe over with a slightly damp rag with a little saddle soap ( just a smear) should do the trick. This should not cause any harm to the leather car seat, but to be on the safe side try first on a small inconspicuous area....
I hope this info is useful to you.... I find it a complete solution with the leather seats on my car.
Here's a link to a website that gives details of the breeches ( just as an example)
http://www.ebay.co.uk/itm/like/200861549401?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=108&device=c&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0&ff12=67&ff13=80&ff14=108&ff19=0
Usual disclaimers apply( I have no connection with these people or manufacturers of breeches/jods etc.
Regards
paws