If in doubt, don't.
The classic treament to keep leather supple. Many 'balms' and 'polishes' contain it. Just take notice of what you are buying - sometimes even when it is labelled 'pure', it is not unadulterated.
Also notice that SOME products sold as neatsfoot oil contain substances that will actually rot stitching made from natural thread - mine is linen. I generally use the 'Carr and Day-Martin' brand (UK). Lincoln also have a good reputation for 'traditional recipe' products. Look for saddlery suppliers and read the labels.
Avoid overdoing it unless you want your leather very floppy (and/or water-resistant). I use a little on every new strap. Apply it VERY sparingly - if you don't, light-coloured leather will take a day or two for it to become invisible again. (It temporarily darkens the surface - or permanently if you overdo it).
This is probably favourite. Sold by a number of companies. Contains softening oils and, often, light waxes. Just be cautious about manufacturer - go to horse tack suppliers again.
The typical horse-tack 'finishing' treatment after the saddle or bridle has had leather dressing applied.
Don't get tempted to actually WASH any leatherwork with this. It is a surface treatment and gets applied with a damp cloth or sponge.
Waxes protect the surface from minor contaminants.
I'm using 'Carnauba Creme' as a finish just now. Fiebing's. Carnauba is a hard wax from a Brazilian palm Copernicia prunifera. Good in car waxes, too. Beeswax is good. Softer. (So, not for your car, maybe).
Even a soft wax, alone, won't sink in far; surface protection, not conditioner.
If you live in a place where you cannot access specialist suppliers, you could do worse than this, DEPENDING on the finish of the strap.
A combination of waxes and oils. Use a decent make. Buff it properly. (Else your clothes will get it). Use Neutral on light-coloured leather... maybe just: USE NEUTRAL full-stop.
Mink Oil is, by all accounts, another classic treatment. Never used it. Trouble is, some of the products described as 'mink oil' seem to contain other fluids - some say silicone - that I might not want on my leather. And then there's the ethics of this one. One entry in Wikipedia suggests that most of the oil comes from China, another that most mink farming is done in Europe. How much oil is there in a mink anyway...
Silicone, by the way, I avoid because it's just about impossibe to remove once used and, as in car waxes, that means surface repairs are compromised. It's tempting because of its water-repellent properties. A lot of walking-footwear treatments use it.
Best to do:
Occasionally ‘feed’ and clean it, but don't overdo it. Just when it starts looking a bit grubby, or feels dry. Even if you do nothing to it other than use it, leather will still last, but it will tend to crack if neglected for a long time.
All sorts of treatments can be used, but bear in mind that they have to soak into the surface to affect the flexibilty and, especially on the light-colour surfaces, will change the appearance to a greater or lesser extent. Waxes applied to the surface don't penetrate far but do help to resist liquids soaking in.
Don't panic merely because the leather darkens a little as you are using the wax 'polish' or other bottled liquid treatments - provided that they have no dyes in them - the effect fades as they dry - and provided you don't over-apply.
(But you'll not be putting dark-tan shoe polish on your light-tan leather. Will you).
Best to avoid:
Please don’t leave leather in direct sunshine.
(During all of those crazed Summer Festivals you'll be playing at).
OK, you can get away with anything once in a while, but a car's parcel shelf is cruelty.
Getting it soaking wet won't destroy it, but will probably spoil my nice shaping.
(You can see here that I'm still envisaging you doing the extreme-environment gigs... )
It will still develop a pleasant, weathered look without you acting savagely.
If you tip a pint of beer over it, it will stain. Now, as you'll know from what happens to leather shoes, a black-dyed item isn't going to show much obvious change, but my carefully saddler's-tan-dyed strap will.
A clean, wet cloth applied immediately will rescue the situation a bit, but you just have to accept that such incidents are going to leave a little mark of history.
Liquids with more serious dyeing properties - red wine, ink, blood... (you do go in for those sorts of games, don't you?), will soak in and stain permanently.
These sorts of accidents will be less life-changing if you maintain a lightly-waxed surface. Obviously I mean on the strap.
Leather is, of course, essentially a natural product, although it is a lot more ‘manufactured’ than many of us might have thought.
I'm compiling some information on manufacture. Meanwhile, see the 'External Links'.
A strap made from heavy leather is durable - probably outlast you and me - and doesn’t require as much maintenance as your instrument, unless you never change the strings. Well, you don’t even need to tune it...
Just like a good pair of shoes, a strap will mature gracefully, given half a chance. Treated badly, it will become decrepit and the two of you might fall out of love.
A clean, water-damped, cotton cloth will keep the damage to a minimum - but make sure that you wipe it right across or you'll get 'tide marks'. If you have a decent horse-tack type cleaner handy, all well and good.
External Links(Open in new window)
Notes on the External Links
Soundtracks on the videos are generally... corporate, but the Clayton's one, for instance, has some classic footage of what it takes to produce the sort of vegetable-tanned leather that I use. (The tan-coloured hides in the floor-pits, not the blue stuff).
P.S. I've just added the Andrew Muirhead link - it's to an article rather than a video.
I'm not sure that they deal much with veg-tan leather, but their advice is still good e.g. don't over-do the care.
There are also some interesting videos on that site. Very professional. Soundtracks are more tolerable, too.