Its inevitable that your soapstone will oxidize when its exposed to oil. This mind you, is not staining the stone. The dark spot is the oil oxidizing the stone, similar to metal exposed to water. Some people choose to speed up the oxidizing process and oil the whole counter with mineral oil or wax. This of course will darken and enhance the veins of the stone. And some people prefer a lighter grey countertop, therefore they just remove the oil spots. Here are a few examples of what to use to easily pick up those marks.

Apply any of these products:
Window cleaner
Disinfectant spray or wipes
Dish or hand soap

Directions:
1,) Wipe on, wipe off
2.) Then rinse
3.) Viola! Should look as good as new!