Well...it finally happened to me. I had been running Havoline or Havoline HM in my old (1990) Jeep Comanche. I recently replaced the Havoline HM 10W30 with Havoline Synthetic 5W30. After 400 miles of driving I have developed a nice leak in the rear of the engine. The Jeep never leaked a drop with any prior oil, so I can only assume that the synthetic cleaned a seal and allowed the leak to occur. In defense of Havoline Synthetic, the Jeep has 160,000 miles and it sat with minimal driving for almost 5 years in my Dad's barn. I have been driving it for 1 year with 2 ARx treatments. The engine runs well and I want to keep it running as long as possible. I chose to run the Hav.Syn because I had it on the shelf and because it has been pretty cold here. The engine runs great with this oil (smooth, nice idle, etc.), but now I have this leak. Would you guys suggest.. - Running the sythetic for another 500 miles to see if the leak stops - Replacing immediately with a HM oil Of course the issue might be that a seal has deteriorated beyond the ability of any oil to stop the leak. In that case I might just have to accept the leak stains on the driveway. Thanks for your advice.