Mother in law thinks formula is poison

There is absolutely nothing wrong with FF. It is Mum's choice how she feeds HER baby; you don't need to justify YOUR choices either; it isn't her business, don't explain why. This has nothing to do with MIL. Don't sneak about, feed baby infront of her confidently with the bottle.
If MIL says anything rude, remind her that it is detrimental to a baby's development for the Mother to neglect her own health and what she is comfortable with; fed is best. Then get your dh to have words to tell her in no uncertain terms to change her attitude, if she wants to come over again.
I wouldn't say anything beforehand, as MIL may not say a thing.

Your husband should phone her, tell her you are mixed feeding and if she's going to be a twat about it she should stay at home

Don't sneak around.

Explain about your oversupply and that medically you've been told to not feed as much because there is a risk to your health. Warn her once that any judgemental comments and you will leave. Then if she makes any comments, leave.

Tell her she doesn't have to drink it and that you're going to feed your baby in whatever way works best for your family

She sounds silly.

The attitude you describe isn't as old fashioned as you think. When my 75 yo mother had her children, post natal wards had a breastfeeding nurse and a bottle feeding nurse who would teach women to feed in their preferred way before they left the hospital. I suspect there was far less rigidity about what mums should do than there has been in the last 20-30 years or so, so it is surprising to me that someone her age is so extreme on this.

Anyway, don't sneak around like you are doing something wrong when you are not. Unless she or your DH proposes to start lactating you need to feed your children however works for you and your family. I'm sure she'd prefer them to be fed than not. If she is unpleasant about it you don't need to visit and I think your DH needs to grow a pair on this one and make clear your shared position.

I chose to never use formula for two of my three children out of ethical objections to formula companies and their marketing, and also because I just hate the idea of it for MY children, but even I think your MIL is nuts. Pretend you don't know what she thinks. If she says anything just smile and nod and ignore.

I would proudly stick the formula tub on top of the Xmas tree, it's none of her bloody business. DH needs to grow pair and not tolerate any BS from his Mother.

Otherwise, get your coat on and head off out for a nice lunch leaving DH with the kids.

If she really pisses you off, take the kids with you.

OP you absolutely don’t need to explain yourself to anyone, including your MIL. She’s not the head of the anti formula task force!!. No one should have an opinion on how you feed your own child. As suggested above, your husband needs to tell her to keep her opinions to herself, otherwise I would not be going to her house, where I would not be welcome to be myself, and comfortable with my baby.

She's mid-70s not mid-90s!!

She might be very wrong with her attitude (she is!)

But she wasn't using watered down tinned milk!

Tell her not to drink it then!

Feed in the way that works for your family. If it's a problem for her, reassure her she doesn't have to drink it.

DH needs to talk to her before she comes.

"Mum, I wanted to let you know that we are combi feeding baby and I know your opinions on formula but I don't want you to make any comments about it while you're visiting. We've made the decision that's best for us and we are happy with it. Looking forward to seeing you"

I did this with my dad when I had tattoos and was going on holiday with him so he was going to see them. He's a notoriously opinionated man but he kept his mouth shut.

Anything negative at all then stand up and leave. Don’t justify or anything, why you are formula feeding is no one’s business.

If it’s got sunflower or any other vegetable oil in it then it’s toxic, coconut oil is better as it’s saturated.

I wouldn’t bring it up as a topic, but I also wouldn’t hide it.

But knowing how MIL is, I’d have some fun with it. I’d get DH to do the first feed in front of her, and when she comments DH should respond asking ‘how exactly he should breastfeed his child given that the nipples his DM birthed him with are useless!?’

Absolutely.

I went with option 1 when my MIL realised I wasn't breastfeeding when we went round to hers when baby 1 was a few weeks old. She 'recovered' from the realisation instantly and happily took the baby and bottle from me and happily did so for every visit going forward. Everyone was happy.

Isn't it a shame, I've seen women hide themselves breastfeeding babies in empty rooms and under covers, and we also have formula feeding hidden by sneaking around. Everyone should be able to feed their kids without comment or shame from other people.

It’s none of her business, and she gets absolutely zero say. You tell your husband that he either tells his mother to mind her own business about how you feed your own child or she stays away from the house. Warn him that if he doesn’t, then the first peep you hear from her will send you nuclear. Follow through on it.

Have to face mother in law next week and DH suggested we just sneak around hiding formula and making it upstairs out of view pretending I am feeding up there

Your husband is absolutely pathetic. Why on earth hasn’t he told her to keep her opinions to herself years ago?

You do know op, you don’t have to face her at all? You can opt out of visits for any reason. You don’t have to join in and be as scared of her as your gutless husband is.

I amn100% for breastfeeding. However, the only perrson I have the right to decide this for is me... I dont judge others on their choice as its their choice.

Everyone else gets to chose for themselves.
Tell mil to stop the comments if she can't say anything nice don't comment.

.don't sneak around.

No sneaking around. When she says something you tell her she had her chance to raise her kids back in the days and she has no say how you TC yours.

As someone who spent the first couple of months formula feeding in hiding, (e.g. in the car), because I was too ashamed to do it in public, I want to ram her face into an open tin of formula powder on your behalf. I realise that’s not helpful but seriously, don’t hide yourself in any way, she sounds like a judgemental old bat and there is nothing to be ashamed of.

Sneaking around making formula so the MIL doesn’t see? No way, absolutely not. You should have told her and your husband to fuck off the first time when they suggested you shouldn’t give it to your other children. Now you’ve just made a rod for your own backs.

I’d laugh in her face and say “interesting view point” with a pronounced eye roll. (I’m quite immature but fine with that).