You could try a Whiting paste... theres videos of people making and using it on milsurp mosin nugents in the usa, pulling out loads of black oil stains.

Mix acetone with chalk powder until you get a toothpaste like consistency, spread it on the affected areas and the acetone should draw the oil into the chalk which you scrape off once dry. Repeat the process until it's all out. Definitely worked for those youtubers!