Leather sporran care

[artificer]

I've just bought an old brown leather sporran off ebay and although it's in great condition I wonder if anyone can recomend something to treat it with to keep it in good condition.

Be VERY careful. Depending on the age of the leather and how much it has dried, conditioning it may actually speed it's breakdown. Very old leather can swell and crack after it's dried too much and then conditioned again.

Do you have an info on the hows and whens of the piece, or can you post a picture of the piece?

Assuming it CAN be conditioned safely, here are a few tips:
1) DON'T buy your conditioner at an auto parts place, or in the auto department of any store (barring a few exceptions I'll mention in a bit). Most creams used for auto leathers are too harsh for a good quality leather.

2) ALWAYS test ANY product on a small section somewhere you won't be able to see in normal use. Allow the area a day or so of sitting after application before you use it on the whole piece. TRUST me, the wrong product on the wrong leather can pull dyes out, destroy the oils/waxes & dry a piece out... just wreak havoc on a leather.

3) I personally like Zymol as a general conditioning creme, but depending on the leather, you might wish to investigate neatsfoot oil or mink oil. It depends a lot on the piece. If you have a leather crafters store like Tandy around, you might wish to contact them, or as Bobsyouruncle mentioned, if you have a saddlery/tack shop nearby, they can probably steer you right. Just make sure to bring the piece in with you.

Would the same treatment recondition leather that is becoming brittle and has flaked off some of its outer finish? Can you rejuvenate leather?

No. Assuming the leather is of good quality, once a leather has dried out and begun cracking, your only hope is a good conservator, if the piece is worth conserving. Otherwise, it's best to let the piece 'mummify' and stay dry. Lower qualities of leather with some form of acrylic topcoat that crack are simply destined for the dustbin.

(as artificer mounts his soapbox)
Leather is one of those products where you TOTALLY get what you pay for (unless you're talking about 'designer' labels (clothing and accessories) where you are paying for the name). A good oil/wax finished leather will last +100 years if properly cared for. Newer finishes, while easier to maintain and 'stain resistant' are essentially plastic. I'm sorry if this seems snotty, but plastic is garbage where leather is concerned. The under-layer will outlast the plastic topcoat by a dozen years, at least.

Barring silicone, which is really only good for weatherizing 'outdoor' leathers, synthetics have very little place in quality leather products.
(dismounts, dusts off, and apologizes for rant.)