Summarize:

Oil-based paints are known to be quite a bit trickier than their water-based counterparts when it comes to stain removal, as water is much less effective in solving them. If the paint stain you're tending to is already dry, you can remove at least some of the mess by taking a knife to the surface material. Scrape a blunt knife along the surface; in doing so, you'll hopefully get the excess that wasn't able to latch itself directly onto the fabric. It is generally recommended you use a blunt knife for doing this, as sharp knives run the risk of causing damage to the pants themselves. Unlike water-based paints, which can be attacked relatively easily with warm water, oil-based paints require specific chemicals to be lifted. While paint removers are the most effective antidote for paint stains, they're not promised to be safe for use on clothing. An oil solvent is your best bet; they're relatively inexpensive and can be bought at any supermarket or arts store. Even if you don't have any paint stains yet, it's a good idea to keep an oil solvent handy so you can fix your clothes as soon as possible if it ever arises Using a towel, dab the solvent lightly and apply it to the stained area of your jeans. Rub the stain in small, circular motions, beginning from the outside of the stain and gradually working your way inward. Attending to the stain in this manner will minimize the risk of the stain spilling over into adjacent areas of your jeans. Applied correctly, the oil solvent should lift the paint. Use a toothbrush if you think the washcloth isn't working as well as it should be. If you think a heavier-duty chemical is needed such as industrial paint remover, it's a good idea to test the chemical on an innocuous part of your pants (such as the inside of bottom pant leg) first before moving in to solve the stain. That way, if it's shown to have damage, the damage will have been caused somewhere relatively harmless and irrelevant. If a chemical scrub hasn't properly fixed the issue, cover the stain with a dab of glycerin and let the pants sit overnight. The active chemical agents in glycerin should work towards dissolving and lifting the paint particles from the fabric. If you don't already have some in your cabinet, glycerin is very easy to get and can be found at just about any supermarket.
Scrape paint with knife if dry. Purchase an oil solvent or paint remover. Apply solvent to stained area and scrub. Cover with glycerin, should the issue persist.