which Vs that

[Tdol]

Not that I know as a rule, and, in fact, 'which' is often preferred in formal style, so I would say not. BTW, 'which' is not a rule in formal language- we can use 'that', and when it makes things clearer because there are other whiches around it can make sense to use 'that'.

[dihen]

so I would say not

I especially used "improper" instead of "not proper" so that you might respond with something like "I would say it is" (meaning "I would say it is improper"), but you didn't?

[Tdol]

I didn't say it was improper because I don't think it is. There is a tendency to favour which/who, but that does not mean that the use of that is improper IMO. I also said that there can be cases where its use makes sense- it can be used to avoid repetition when there are many relatives close together, or near other examples of who/which. The 'would say not' referred to your first question.

[dihen]

I believe some American users use 'that' for restrictive and 'which' for non-restrictive...

Not that I know as a rule...

Do you mean that British English speakers don't often only use "that" for restrictives?