Leather needs to breathe, as do your hands. Mink oil is the best for "waterproofing", but will eventually soak thru as well.
A better solution is an overglove, such as for snowboarding.
Once, and I know this sounds ridiculous, I used surgical gloves;yep, the kind you pull over your hands when doing dirt and grease work on the bike or car.
I had a pair in my bags; it was freezing rain and I put them on (under my gloves) to help keep my hands dry and warm. I imagine they could also go over the glove although I've never tried it and I'm sure they wouldn't last long before tearing. Either way, always good to carry a pair or two.
On boots i use Mink Oil and maybe once every two years on my jackets. On everything else i only use Doc Baileys leather tonic. Its a leather conditioner and wont leave a oily feel. I use it maybe twice a year on my jackets and gloves.
They do offer a "Rain Formula" but i have never tried it.
I use surgical type gloves made for mechanics. I think they are slightly thicker and more durable than normal ones. I only use them as a backup though since they are easy to store.
I found a pair of neoprene fishing gloves, kinda like wet suit gloves. I use them mainly for trout fishing and even after sticking your hand in freezing water OVER the glove its not bad except they will get funky sometimes. They worked surprisingly well one day under some plain leather gauntlets but you still get kinda wet, just not as cold.
I use Huberds shoe grease on my gloves, boots, leather pants and leather jacket with great results. I lived in Oregon for 48 years and have rode all day long in heavy rain with good results from this product.