A&M,
It seems likely that the oil has already been attacked with sufficient gusto - what remains is probably largely a carbon deposit.
Though Eco Spotter will often remove this, I would try Prochem A217 Ultrapac Renovate, applied fairly warm (not hot), lightly agitated, given a dwell time of at least 10 minutes and re-agitated prior to extraction with an acidic rinse.
Remember that the pre-test will require the same dwell time