Get some Simple Green. Simple Green is a concentrated liquid cleaner that you can buy in discount, hardware, automotive and a lot of grocery stores. It's non-toxic and biodegradable also. It will not bleach the color out of your jeans either.
Take the jeans and spray the Simple Green on them, cover the entire area well. Let the cleaner work for a good hour, go back and rinse the area with hot water. It it appears that most of the stain is gone, spray it again, leave it lay for awhile and then launder as usual. If the stain has lessened but is still pretty visible, give it another good spraying of the cleaner and let it soak awhile more. Keep treating the spot until you can hardly see it and then into the washer.
I've used Simple Green to take axle grease out of jeans and with a little patience you should be able to get the motor oil out.