I was towing my 98 wrangler (4.0 - 6) 4 wheels down per manual. After @ 15 min. I noticed lots of blueish white smoke from behind it. I pulled over and couldn't find anything wrong. Brakes were cool. After I re started I drove @ 160 miles over 3 hours with no smoke no problems. It ran smooth. After a couple of hours I started down the road and lots of smoke started blowing out the exhaust pipe. Started it back up @ 10 min. at idle with no smoke. There's spotty oily mess under hood but oil level is full. Any ideas ???