You talking about upholstery or carpet in the car? There are products in the car shops. Heavy oil can be removed with a solvent like dabbing on varsol or gasoline. Careful as both are flammable. No smoking. Vent the car after done. Use a soft old toothbrush to work up the suds if you use a spray carpet cleaner product. Dish wash detergent works pretty good on carpets and rinse it off later.
The best way to get rid of stains in your car will be determined by the stain in your car as different stains require different stain removal. You can use car soap and a sponge to get rid of the stains. Visit some of the local auto shops to get the best stain removers.