Very true. My company is very good about getting oily stains out but it's all self made propitiatory solutions.Stains like that may take a couple of cleaning efforts over time to pull the oil out of the fabric - the oil will collect dirt so the stain will seem to get darker over time but each time you clean it - your getting more of the oil etc.
Dito.Our head detail guy at the dealer I worked at back in the college days - used a DAWN dish soap as his first step at cleaning a cloth seat or the carpets.
For seats - get the dawn foamed up really good - and simply put the foam on the seat the object of the game is to not get the seat super wet. In most cases light dirt and soda spills will get lifted out with some blotting of a clean rag and dawn soap foam. Then hit it with the shopvac.
Very high alkaline product (11.2+) might not harm in car synthetics but I would advise against this product for other upholstery.Dito.
You can also use Purple Power concentrate (parts stores sell it, its a degreaser) and break it down 50-50 for heavy stains, scrub it with a stiff brush then vac. Lighter spills, 20-80. Don't be afraid to get it soaked. The detergent needs the water to work.