Yes, you could use the leather from the sofa to upholster your car seats. No, it doesn't have to be new to use it in a car. Be aware that most "leather" furniture does not have real leather on the outside arms or the outside back, that is usually just matching vinyl. The leather is only on the seating areas and the arms. The same is true of most production line leather car seats.
To do a pair of bucket seats, or one bench seat, you would need the equivalent of about one full cow hide, which is about 45 to 55 square feet of leather. That would be the same as 2 1/2 to 3 yards of fabric. It's extremely doubtful you could salvage enough useable leather from the biggest totally leather sofa to do an entire car interior.
Neatsfoot oil is not for use on upholstery leather.
No one lives forever, the trick is creating something that will.
Last edited by DanTwoLakes; 09-18-2012 at 08:39 AM.