Any of the well known hand cleansers used by auto mechanics or similar are excellent for cleaning oil stains from clothes or even worse carpets. The stuff I use on my hands after I do work on my lawn mower is excellent and seems to remove even the worst grime. Some of these cleansers contain hydrocarbon solvents which may remove colour from clothing, so try using some on the tail of the shirt first to see what the affect will be. Wash the shirt in your washer after removing the majority of the stain. Cotton is a somewhat porous fabric compared to to synthetic fibres which are more smooth. So the oil content of the stain will be removed, however you may not be able to remove all the stain, especially if black, waste oil is involved. However it may become less obvious after further washes.
