One lane in a milelong stretch of Interstate 5 on the downhill side of Roberts Mountain will remain closed much of today after a southbound freight truck crashed overnight and spilled industrial resin. The accident occurred about 10 p.m. Wednesday near Milepost 115. Crews worked overnight to clean up the mess. Transportation officials urge motorists to slow down coming over the mountain, where the speed limit is posted at 65 miles per hour. Oregon Department of Transportation spokesman Jared Castle advised motorists headed to work in Myrtle Creek, Canyonville and Riddle to give themselves an extra five or 10 minutes …