Kwalin mai nauyi ne.

No. This is a different mai.

In Hausa, mai can also be a word meaning having, with, or characterized by. So in mai nauyi, it does not mean oil. It is part of a descriptive expression.

Literally, mai nauyi means something like having weight or one with heaviness. In natural English, that is just heavy.

This is a very common Hausa pattern:

mai + noun → an adjective-like meaning

So:

mai nauyi = heavy
nauyi by itself = weight / heaviness

By itself, nauyi means weight or heaviness. It is not exactly the adjective heavy on its own.

To get the meaning heavy, Hausa often uses the phrase mai nauyi, literally with weight.

Ne is the word that helps make this a full statement like is in English.

So:

Kwalin mai nauyi = the heavy box
Kwalin mai nauyi ne = The box is heavy

In this kind of sentence, ne marks the predicate and completes the idea. It is very common in Hausa equational/descriptive sentences.

Because Hausa does not always use to be the same way English does.

In a sentence like this, Hausa can put the descriptive phrase first and then use ne at the end instead of inserting a separate verb in the middle. So the structure feels different from English, but the meaning is the same.

By itself, kwalin mai nauyi is just a noun phrase: the heavy box.

Once you add ne, it becomes a full sentence:

Kwalin mai nauyi ne = The box is heavy

So ne is what turns it into a statement.

With the mai + noun pattern, the descriptive part normally comes after the noun in Hausa.

So Hausa says:

kwalin mai nauyi
literally: box having weight

not something shaped like heavy box with the descriptor first.

So for this kind of description, the order is:

noun + mai + noun

No. It is a very natural and useful way, but it is not the only one.

Another common way is:

Kwalin yana da nauyi
literally: The box has weight

That also means The box is heavy.

So mai nauyi is one common descriptive pattern, not the only possible wording.

Often, ne changes to ce with a feminine singular noun.

For example:

Motar mai nauyi ce = The car is heavy

So one thing to notice is that the final copular word can change depending on grammatical gender.

A natural negative version is:

Kwalin ba mai nauyi ba ne.

This uses the Hausa negative pattern ba ... ba around the descriptive phrase. So the basic idea is:

ba + predicate + ba ne = is not ...