Lamb Shearling, care for (or not)

I have a really nice, black, high quality lamb shearling coat.

The seller, a fur/leather shop of great age and reputation, suggested I use Nivea [sic] to care for the coat by applying small amounts on a monthly [sic!] basis.

The Internet seems to say that...
* Do not use Nivea!
* Do absolutely nothing more than 'general get rid of dirt' care
* Use neatsfoot - or, do NOTuse neatsfoot
* Use minkoil - or, do NOT use it at all
* Use Saphire Renovateur
* It is lamb, some lanolin cream cannot hurt

What I want to achieve is:
* Leather not so dry that it cracks
* Longevity and durability

I use the coat also in small amounts of rain for short time, and it can then airdry.

Is there any more authoritative answer long-term care for fine lamb shearling, especially, keeping it from becoming so dry it cracks easily?

I think that most people on this forum would agree that in most situation you should do nothing.
The majority of leather jackets don't need any sort of attention for the first few years of their lives, sometimes decades.
If we are talking about a MC jacket that gets used in the sun, in the rain, and gets covered in bugs and road grime, then we are talking about something different, but a "regular" leather jacket doesn't need much care at all, especially when new.
The idea that you need to treat it monthly is a way to sell you conditioners, not a leather need.

I would say don't use Nivea, it has a bunch of creamy stuff in it that has nothing to do with leather.
I have seen pictures of jackets that had a white sticky film on them after Nivea treatment, don't do it.

A lot of people hate on neatsfoot and minkoil, i have used both for years without problem, but i use them both on things stitched with poly thread (belts, wallets, etc). It has a reputation for being bad for cotton thread stitching, i can't confirm or deny. But it shouldn't have ill effects if you don't have cotton threads.

Now if you decide you still want to treat your jacket, the two gold standards are Vaselin and Pecards. Vaseline is recommended by John Chapman from Goodwear leathers and has been used by many to restore vintage jackets.
Pecard's is what i use on older jackets and it seems to work well.
I don't treat jackets regularly, i only use Pecards when i get an old vintage jacket that feels dry. I have also used it with succes on new jacekts that were not waterproof.

Now to talk specifically about shearling, it is such a porous hide, i think i would keep it dry. There are still old B-3 from WWII going around, most of them have most likely never been treated and are still going strong.
I would just wear it, make sure to dry it naturally if it gets wet, and enjoy it.
My Schott B-3 for example is 25 years old and has never been treated with anything. It doesn't feel dry at all.

@Dav has a bunch of shearling jackets, he should know better...

If it has a painted on top coat, think ELC, then I know a drop of Obernauf oil or LP does no harm at all, I mean a drop though. I did my Irvin with it last week, just dipped the end of two fingers in the oil, rubbed around my hands and then massaged it into the hide, not sure it did much, but I assume it'll prevent some cracking of the top coat.
If it has no top coat then I'd leave it alone as cream or oil may leave a stain.
As mentioned above, Picards is also very good, I've not tried vasaline on a jacket but have put it on boots very successfully.
In general though they don't need much care if you're wearing them, long term storage is where drying in likely to occur.
Edit
I only did the Irvin as I had the oil on my hands from treating some boots already, it didn’t really need it but it seemed a shame to wipe it off on a rag, so I used the jacket instead.
Incidentally, I put it in the washer over the summer, no soap, and it still didn't really need treatment.

Care tag on older Avirex sheepskin / shearling jackets recommends lanolin, which makes sense, I guess. Lanolin is okay for leather but applying it isn't easy.

Either way, different rules apply for shearling, for all the obvious reasons.

If the coat is suede-ish, you technically can treat it with either lanolin or any quality leather conditioner like Pecards, Obernauf, etc. but as @Dav said, this might lead to stains that might not even go away, especially if the conditioner you're using is greasy (like Pecards).

If the surface of your coat is smooth and shiny, then there isn't much you can do. Thing is, it's shiny because it's basically been plasticized. Meaning not much of anything will get through to the actual leather.

Long story short, treating shearling isn't simple.

What you can do though is to find a good water based conditioner, like Lexol and get it in a spray bottle. It's how I'm treating my shearling jackets. That way you can safely use it for both sides of the coat - if the coat has a shiny finish, spraying the furry side is the best way to treat the leather - and in my experience, even if it stains, the color will revert back to normal once it dries up.

And yeah, I'd definitely avoid Nivea.

Someone with too much time on their hands should get a bunch of scrap shearling swatches and apply various treatments and cleaning methods to them and see how they do, and publish the results.