I was just talking to an old mechanic friend of mine who still works on 60's european cars. We had always known that switching to Redline ( highly desirable in aircooled cars) could cause leaks. He has found that gradually adding Redline oil, say 25% on the first oil change, 50% the second, never leaks. He also thinks it is best to add a little Redline through the break in process, as this has prevented any leaks from developing when the car goes to 100% Redline.