If they are expensive trousers, you could ask the manufacturer.I have heard that toothpaste takes out grease stains, but have never tried this. You could also try neat liquid clothes detergent , the clear kind put on before washing and gently agitated, this has worked for me before. Whatever you do try, patch test on a seam or inside hem first as the cure can be worse than the complaint!!The big point is to avoid heat, so luke warm wash do not iron or tumble dry as this sets the stain.