last updated 8 Nov 2018

Under Construction There are dozens of leather treatment products.
Here's a summary of some common ones.

Guidelines:
Moisture can promote further deterioration for leather bound books and some other antique leather. So you may not want to use products containing water.

How To Clean, Condition & Polish Leather | Conditioners, Oils, Lotions, Weatherproofers And Polishes For Leather | RealMenRealStyle.com

1. Clean:
Saddle soap can be used to remove dirt from exterior of leather. However, A lexol leather cleaner would be a better choice. A saddle soap will most likely be too strong for the leather on your bag and could cause discolouring.

A regular soap will be very drying and a saddle soap (which was originally designed to soften leather) has a high oil and fat content.

Recommended:
Chamberlain's Straight Leather Cleaner
Leather Cleaner by Leather Honey
Lexol Leather Cleaner
Saddle soap

2. Condition:
Mink oil, leather honey and neatsfoot stimulate the natural oils of the leather.

Recommended:
Leather Honey leather conditioner
Chamberlain's leather milk
Lexol Leather Conditioner
Armstrong's All Natural Universal Salve
DannyB1 at Favorite Leather Conditioner?? | Styleforum says,
I've used both lexol and leather honey. I have not noticed a a big difference between them in terms of results. Lexol is much lighter, leather honey is very thick, but both seem to make leather feel th same. I mainly use them on shoes and bags.

althanis said, I used Obenauf's LP on a horsehide briefcase, and was annoyed that it became sticky and would transfer to my trousers.

Patric BOOTH said, If you're going to use Obenauf's on briefcases, or furniture or whatever you should use their Leather oil.

3. Protectants:
Mink oil
Neatsfoot oil
Beesbutter
Meltonian brand of shoe creams
Tarrago Shoe Cream
Sno-Seal Original Beeswax Waterproofing
Jobsite's Grizzly Grease


Replacement:
The leather straps on my Grandparents old trunk were already broken and beyond repair. I called around tack shops, etc and they all recommended Tandy.
They only had two sizes of strap one that was slightly to thick and one too thin. I got a 6 foot thick one and found it was thinner at one end than the other, so if you need a strap go thru their rack and see if you can find one the right size.
Etsy also has straps in 3/4" wide leather. 30, 60, 72 & 80 inch lengths.
Common recommendations: neatsfoot oil, mink oil, coconut oil
How to Restore Antique Leather | Hunker
Never use furniture polish, car care products, or any oil, wax,
 or silicone-based leather conditioners not formulated
 specifically for antique leather.

Best Rated in Leather Care Products & Helpful Customer Reviews - Amazon.com

x.x - score (nnn) Number of reviews

Obenauf's Leather Oil Condition Restore Dry Leather 4.7 (687)
It is a natural product using  propolis and Beeswax.

Obenauf's Heavy Duty LP Leather Conditioner Natural Oil Beeswax Formula (8oz): Automotive
FRYE Leather Conditioning Cream in Neutral: Shoes 4.7 (547)

Chamberlain's Leather Milk 4.5 (1025)

Lexol E300858000 Leather Deep Conditioner, 4.6 (998)

Fiebing's Aussie Leather Conditioner 4.8 (111)

Leather Honey 4.4 (4,902)

 Griot's Garage 11141 Leather Rejuvenator - 8 oz.: Automotive 4.3 (71)

See also Best Leather Repair Kit Products Online for Cars and Furniture - Furniture Clinic

Links:
Best Rated in Leather Care Products & Helpful Customer Reviews - Amazon.com Restore your Vintage and Antique Leather - Preservation Solutions Best Leather Repair Kit Products Online for Cars and Furniture - Furniture Clinic 3 Extremely Effective Tips on How to Soften Leather Favorite Leather Conditioner?? | Styleforum