
Noooooooo..... dont soak leather in oil!
Not good for it, if its dry you will make it go like its been fried - if its not dry it will loosen off the fibres so that it goes all spongey and weak and horrid.
Use a smear of oil on yoru hands if you must, or a smear of one of the grease/fat based leather softeners again on your bare hands.
Run the lead through your hands and let the warmth of your hands massage the stuff in and ease the stiffness out of the leather carefully (dont go mad bending and twisting it!).
When you want to clean dog leads/collars etc - warm water TINIEST drop of washing up liquid (no where near enough to make foam). Clean cloths - dip the cloth and get the excess water off so its not properly wet but also not bone dry. Rub the leather well especially the flesh side and any areas that are against the dogs fur and get greasy. Check any stitching for wear and tear (can be sorted out easily and cheaply if its hand sewn - if its machined or rivetted yer out of luck and that will teach you not to be so skinflinty in future when a lead is nice and soft... but the horrid machined stitching or rivetts have ruined the ends pahpah spitspit etc! /grump).
Dry the now damp but clean leather with a clean soft cloth, and then rub in a leeeeeeeeeeeeeeeeeeeeetle smidgen of leather conditioner or if its something that gets wet a lot, i like Gwax leather waterproofer (i like some really foul smelly stuff i made myself also but thats a secret and involved me nearly poisoning myself melting things over the cooker ew!) a lot, and also Sedgewicks leather balm.
Polish with a clean cloth to finish.
Do not - on pain of death - soak leather in oil ; Soak leather in hot water; Put wet leather to dry in the airing cupboard or on a radiator or in the sun (but mouldy leather wouldnt go amiss in teh airing cupboard as long as thats dry and not damp, for a leeeeetle while) or anything else silly.
Mostly you cant soak modern leather in oil because its too thin and lightweight and not dense enough to survive it. 50 years ago (less even), our quality english leather was heavier and thicker and denser, partly because our cattle lived a fair bit longer t han they do now and did not grow so quickly either.
Cheap nasty leather is even worse and if you soak 'leather board' in oil it will probably disintegrate. (Much of the cheapest dog leads are made out of verrrrrrrrrry nasty leather).
Here endeth the sermon on leather. *gets off soapbox... shuffles off*