Postino,
I spray on Simple Green and lightly scrub with a soft toothbrush and rinse with water. I only do this to the outside of the grips if there are any stamps on the inside. I may repeat several times depending on how dirty the grips are. I really don't let them sit and soak. Simple Green dissolves oils and grime much faster than Murphy's. I pat dry with a soft towel and then put them in direct sunlight for several hours. I've read that a blow dryer also works. Don't freak out
when the color changes to gray. You don't want any moisture/water left on the grips when you apply boiled linseed oil very lightly and pat dry. Let them sit overnight, and the oil will penetrate and some will evaporate.
It works for me.