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Can I say,

(a) He filled up / on / fill in the petrol at the petrol station.

(b) He filled the car with some petrol oil.
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Vincent TeoCan I say,

(a) He filled up / on / fill in the petrol at the petrol station.

(b) He filled the car with some petrol oil.
As a Yank, I'm more accustomed to "gas". I don't believe the Brits use 'petrol' and 'oil' together, but I could be wrong.
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You are right. Petrol and oil are two different things, albeit both stuff you have to put in your car.
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So, what are the answers?
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Vincent TeoSo, what are the answers?
He filled the car with petrol. (BrE)

He filled the car with gas. (AmE)