You don't say which year this is, but it's it a direct injection engine? Some GN can suffer LSPI low speed pre-ignition. Pinking in old money. This can damage the pistons, chin rods or cylinder heads. All of which could give the oily spark plug and poor running/EML
ASTRA 1.4 t have this problem but don't know if it's the same engine on your family car.
It's worse in low rev acceleration.
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