Por que a troca de óleo custa muito?

Because por que is used in direct questions when it means why.

In Por que a troca de óleo custa muito?, the speaker is asking a question, so por que is the standard form.

A quick comparison:

• por que = why / for what reason
• porque = because
• por quê = usually why at the end of a question
• porquê = a noun meaning the reason

Examples:

• Por que você saiu cedo? = Why did you leave early?
• Saí cedo porque estava cansado. = I left early because I was tired.

Portuguese uses articles more often than English does.

So a troca de óleo literally looks like the oil change, even if in natural English we might simply say an oil change or just oil changes depending on context.

Here, a is the feminine singular definite article because troca is a feminine singular noun:

• a troca
• uma troca

Portuguese often treats this kind of service or procedure as a specific noun phrase with an article.

Because troca means change or replacement, and troca de óleo is the normal expression for the service of changing a car’s oil.

It is a set phrase in automotive contexts.

Even though mudança can also mean change in some situations, mudança de óleo would sound unnatural here. For car maintenance, the usual expression is:

• troca de óleo = oil change

Similar service expressions use the same pattern:

• troca de pneus = tire change
• troca de filtro = filter replacement

Because de óleo is being used in a general, descriptive way: oil change as a type of service.

In Portuguese, when one noun describes another in this kind of general expression, you often use de without an article:

• troca de óleo = oil change
• garrafa de água = bottle of water
• sapatos de couro = leather shoes

If you say do óleo, it usually sounds more specific, as in the oil already known in the conversation.

So:

• troca de óleo = oil change in general
• troca do óleo = the changing of the oil, a specific oil already being referred to

Because the subject is a troca de óleo, which is singular.

The main noun in the subject is troca, and it is singular, so the verb must also be singular:

• A troca de óleo custa muito.
• As trocas de óleo custam muito.

Even though óleo appears in the phrase, it is not the subject. It is just part of the expression troca de óleo.

Both are possible, but they are structured differently.

• custa muito = costs a lot
• é muito caro = is very expensive

So:

• Por que a troca de óleo custa muito?
• Por que a troca de óleo é muito cara?

Both are natural.

Notice the agreement difference in the second version:

• caro changes to cara because it describes troca, which is feminine:
  • a troca é cara

But muito in custa muito does not change, because there it works like an adverb modifying the verb custa.

Because in this sentence muito modifies the verb custar, not the noun troca.

So here muito means a lot, and adverbial muito does not change:

• custa muito = costs a lot
• trabalha muito = works a lot
• chove muito = it rains a lot

But when muito describes a noun, it must agree:

• muita troca = a lot of change/replacement
• muito óleo = a lot of oil
• muitas trocas = many changes

So in your sentence, muito is correct.

Yes. That is a very natural alternative.

The difference is small:

• custa muito focuses on the amount of money paid
• é muito cara focuses on the service being expensive

In everyday Brazilian Portuguese, both are common and natural.

Just remember the adjective agreement:

• a troca → cara
• o serviço → caro

Yes. This is the most normal word order.

Portuguese often forms questions just by using question words plus normal statement order:

• Por que a troca de óleo custa muito?

This is very similar to:

• Por que ele saiu?
• Quando você chega?
• Onde ela mora?

Unlike English, Portuguese does not usually need a helping verb like does.

So English:

• Why does the oil change cost a lot?

Portuguese:

• Por que a troca de óleo custa muito?

No equivalent of does is needed.

You normally would not say it that way in neutral speech.

The usual form is:

• Por que a troca de óleo custa muito?

When why comes at the end, Portuguese usually uses por quê with a circumflex accent:

• A troca de óleo custa muito por quê?

This can happen, but it is less neutral and often sounds more emphatic, surprised, or conversational.

So for learners, the safest choice is to put por que at the beginning.

The accent mark shows the stressed syllable and helps indicate pronunciation.

óleo is pronounced with stress on the first syllable: Ó-le-o.

Important points:

• The ó is stressed.
• The word has three syllabic parts in Brazilian Portuguese: ó-le-o.
• The accent is not optional; it is part of the correct spelling.

Without the accent, the reader would not know the standard stress pattern correctly.

Yes. Custar can also mean to be difficult or to take effort.

Examples:

• A troca de óleo custa muito. = The oil change costs a lot.
• Custou entender isso. = It was hard to understand that.
• Custa acreditar. = It is hard to believe.

But in your sentence, because the subject is a service and muito refers to money/value, the meaning is clearly to cost.

They mean different things because the subject changes.

• a troca de óleo = the oil change service
• o óleo = the oil itself

So:

• Por que a troca de óleo custa muito? = Why does the oil change cost a lot?
• Por que o óleo custa muito? = Why does the oil cost a lot?

The first asks about the service price. The second asks about the price of the oil itself.

That is an important distinction in Portuguese, just as it is in English.