Finally, I question I'm somewhat qualified to answer! If you can find a dry cleaner located near you that uses petroleum solvent, then dry cleaning the piece of carpet will remove the stain. I recently dry cleaned a car cover for a customer of mine (I own a dry cleaners here in Texas) who had spilled oil all over the cover in the garage. It came out looking more or less new. There is no reason why the carpet piece shouldn't be able to fit in a standard dry cleaning machine; you just have to find a dry cleaner nice enough to run a single-item load.....which may be hard (insert negative experience at a dry cleaners here). At any rate, good luck!

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Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013